Cholinergic and noradrenergic modulation of thalamocortical processing

CholinergRand noradrenergRmodulationof thalamocortical processing David

A. McCormick

During periods of drowsiness and synchronized sleep, thalamocortical neuronal activity is dominated by rhythmic oscillations. The shift to waking and attentiveness is associated with an abolition of these rhythms and a marked increase in neuronal responsiveness to synaptic inputs. These shifts in thalamocortical processing are controlled by ascending modulatory neurotransmitter systems of which the cholinergic and noradrenergic components play a key role. By altering the amplitude of specialized potassium currents in thalamic and cortical neurons, acetylcholine and norepinephrine can block the generation of thalamocortical rhythms and promote a state of excitability that is consistent with cognition. It has been known since the discovery of electroencephalography (EEG) over a century ago that the frequency and ampfitude of electrical potentials generated by the forebrain vary with an animal's state of arousal (reviewed in Ref. 1). For example, during periods of slow-wave sleep there is a relative synchrony of rhythmic cortical and thalamic synaptic and neuronal activity, while during periods of waking and attentiveness, this activity is desynchronous due to ongoing and complicated processing (reviewed in Refs 2, 3). The finding of Moruzzi, Magoun and others that brainstem stimulation results in EEG desynchronization led to the proposal of an 'ascending activating system' which controls the excitability of neurons in the forebrain4. The anatomical, physiological and pharmacological search for lower level nuclei that could affect wide regions of the forebrain resulted in four candidates. (1) Cholinergic projections from the pedunculopontine and lateral dorsal tegmental nuclei in the brainstem and the basal magnocellular nuclei in the basal forebrain5. (2) A noradrenergic projection from the locus coeruleus6. (3) A serotonergic projection from the dorsal and median raphe 7. (4) A histaminergic projection from the posterior ventral region of the hypothalamus8. These four systems possess a number of similarities: the presence of neurons that discharge at a slow, regular rate that becomes faster (on average) as the animal becomes more alertg-tz; widespread projections to the forebrainS-8; and actions that are slow in onset, long in duration, and that modulate the excitability of the postsynaptic neuron 13-21. Due to their widespread influence and their slow modulatory synaptic actions, these four systems have been implicated in the regulation of a large number of behaviors and behavioral deficits, including Alzheimer's disease, epilepsy, sleep-wake cycles, and cognition, to name but a few. A great deal of information is available concerning the physiological consequences in the cerebral cortex and thalamus of activation of the cholinergic and noradrenergic systems, while investigations of the histaminergic (HA) and serotonergic (5-HT) pathways have lagged. For this reason, this review is focused TINS, Vol. 12, No. 6, 1989

on the postsynaptic actions of acetylcholine (ACh) and norepinephrine (NE). Actions of ACh and NE in the cerebral cortex The entire cortical mantle is densely innervated by cholinergic fibers from neurons in the basal magnocellular nuclei5 and noradrenergic fibers from the locus coeruleus6. Both ACh and NE have potent and longlasting effects on pyramidal neurons in the neocortex and hippocampus 13-2°. ACh reduces three distinct potassium currents in these cells: IM (muscarinesensitive); IAHe; and a resting 'leak' potassium current, IK,l (Fig. 1A)15'19'z°. The muscarinesensitive current, IM, is a voltage-dependent potassium current that is slowly activated (tens of milliseconds) by depolarization of the membrane potential positive to - 6 5 mV (Ref. 23). IAHpis alSO a slow potassium current, although it is not activated directly by membrane depolarization (as is IM), but instead is sensitive to increases in the intracellular concentration of free calcium ions, [Ca2+]i (Ref. 22). IAHPis responsible for the slow afterhyperpolarization (hence its name) that follows trains of action potentials in some regions of the brain. Therefore, IM and IAHp are both increased by neuronal activity: IM because action potential threshold (approximately -55 mV) is within the activation range of the M-current and IAHP because synaptic and action potentials increase [Ca2+]i. Increasing either of these potassium currents reduces the discharge rate of pyramidal neurons (spike frequency adaptation), even during the injection of a constant depolarizing current (Fig. 2A)~3. Therefore, transmitter-induced reductions in Iu and IAHP selectively enhance trains of EPSPs both by decreasing spike frequency adaptation (Fig. 2A) and by facilitating those excitatory inputs that bring the membrane potential near firing threshold15. Inhibitory PSPs or isolated EPSPs will be less affected by reductions in these currents (especially IARp), although they may be enhanced by reductions in IK,l. Application of ACh to cortical pyramidal cells can result in marked inhibition of spontaneous and evoked activity24. However, these inhibitory responses appear to result from the excitation of intrinsic inhibitory interneurons (Fig. 1A, B) 15, indicating that ACh is a purely excitatory agent in the cerebral cortex. Application of NE to neocortical pyramidal cells can have both inhibitory and excitatory effects on spontaneous neuronal activity. The long duration and weak inhibitory effects of NE are mediated by [~adrenoceptors, while the purely excitatory effects are mediated by oc-adrenoceptors and are prominent in mid-cortical (IV, upper V) regions24'25. Intracellular investigations reveal that although NE may cause a small hyperpolarization, its most pronounced action is a strong reduction of spike frequency adaptation (Fig. 2A) that results from a 13-receptor-mediated block of lAHp13,14,18. Together, the weak inhibitory effect and

© t989, ElsevierSciencePublishersLtd,(UK) 0166- 2236/89/$0200

DavidA. McCormick is at the Sectionof Neuroanatomy, Yale UniversitySchoolof Medicine, 333 Cedar Street, New Haven, CT06510, USA.

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ACh Retina Fig. 1. Postsynaptic actions of acetylcholine (ACh) in principal neurons (left) and interneurons (right) in cerebral cortex and thalamus. (A) Application of ACh to a visual cortical pyramidal cell results in a barrage of GABAergic IPSPs followed by a slow depolarization due to a reduction of IM, IAHPand IK,I. (B) ACh rapidly excites GABAergic cortical intemeurons. The increase in conductance, G, associated with the depolarization implies that an increase in Gcat,o, may be involved. (C) Appfication of ACh to LGNd relay cells results in a rapid depolarization (increased Gcat~o,)triggered by the activation of nicotinic receptors followed by a slow depolarization (decreased GK) mediated by muscarinic receptors. (D, E) ACh hyperpolarizes GABAergic neurons in both the nRt-PGN (D) and LGNd (E) by activating a potassium conductance through muscarinic receptors. The perigeniculate nucleus (PGN) is that part of the nucleus reticularis that is adjacent to, and interconnected with, the LGNd. The arrangement of synaptic elements in thalamic glomerufi is shown in schematic form (left, inset). Retinal terminals form tri-synaptic relations with the dendrites of relay cells and intrinsic inhibitory intemeurons (reviewed in Ref. 42). Chofinergic terminals synapse on the dendrites of both the relay cells and intemeurons 3° Time base in each intracellular recording is different: hyperpolarizing current pulses were defivered once persecond. Voltage calibration m (D) is forall traces. (A and B taken, with permission, from Ref. 15; C and E from Ref. 34; D from Ref. 33.) (Copyright for C, D and E: (~) 1989 MacMillan Journals Ltd.) In the visual cortex, ACh application results in an increase in discharge rate elicited by RF stimulation, often without a decrement in selectivity (e.g. orientation or direction tuning) or a significant change in spontaneous activityz6. Similarly, in the somatosensory cortex, ACh application enhances responsiveness by lowering the intensity of somatic stimulation needed to elicit a given response and may also increase the receptive field size. In some neurons, application of ACh even reveals a previously 'hidden' receptive field27. The most common response to local application of NE in the cerebral cortex is a decrease in the rate of Effects of ACh and NE on cortical neuronal spontaneous firing while evoked responses are either r e s p o n s e properties The effects of ACh and NE on neuronal responses only slightly suppressed or even enhanced. Indeed, in to receptive field (RF) stimulation have been tested some cases, application of NE, like ACh, can result in visual, somatosensory and auditory cortical in the appearance of a previously 'hidden' RF regions 26-~8. The most common response to both of response 28. When comparing these results with the known ionic these agents is an increase in the response to RF stimulation in relation to spontaneous firing rates (i.e. actions of ACh and NE in the cerebral cortex, it is clear that these two systems are useful not merely for they enhance the 'signal-to-noise' ratio).

the reduction of spike frequency adaptation may be important factors that underlie the increase in socalled 'signal-to-noise' ratio which is characteristic of noradrenergic action 7. Interestingly, HA and 5-HT, as well as ACh and NE, can block spike frequency adaptation of individual pyramidal neurons in rodent and human cerebral c o r t e x 14'16 (McCormick, D. A. and Williamson, A., unpublished observations), thereby forming a common mechanism by which these ascending systems can facilitate cortical neuronal responsiveness.

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'excitation' or 'inhibition' but rather for fine tuning cortical neuronal responsiveness, perhaps to allow processing to occur in the most efficient manner possible given the present behavioral demands. A c t i o n s o f A C h and N E in t h e t h a l a m u s

Any consideration of how ascending systems modulate forebrain activity must take into account their actions not only in the cerebral cortex, but also in the thalamus. The thalamus is critical for the generation of certain types of forebrain rhythmic activity and serves as an intermediary between the external world and the neocortex. Indeed, many of the alterations that occur in the forebrain processing state (reflected by changes in the EEG) represent the mode of generation of action potentials in thalamic neurons, which in turn is under the control of modulatory neurotransmitters from the brainstem and basal forebrain. The thalamus contains three main cell types: (1) thalamocortical relay neurons; (2) GABAergic neurons within each nucleus; and (3) GABAergic neurons of the nucleus reticularis (nRt; Fig. 1)2. Of the many thalamic nuclei, ACh and NE actions have been investigated in detail only in the dorsal lateral geniculate body (LGNd) and in the nRt. The LGNd is densely innervated by cholinergic fibers arising from

the pedunculopontine tegmental nucleus and the parabigeminal nucleus29'3°, while the nRt receives cholinergic projections from both brainstem and basal forebrain nucleial. Noradrenergic fibers from the locus coeruleus innervate both of these nuclei, although the density of innervation of the LGNd varies with specieszg. Acetylcholine has both excitatory and inhibitory effects in the thalamus, depending upon the type and location of the postsynaptic cell, and even the species of animal studied3e. In the cat LGNd, ACh directly excites relay neurons (both X and Y) due to a nicotinic receptor-mediated increase in cation conductance (Gcation) followed by a muscarinic receptor-mediated decrease in potassium conductance (G~:; Fig. 1C)3e. Interestingly, ACh has exactly opposite effects on thalamic GABAergic neurons (those of the nRt and local circuit interneurons). In these cells, ACh causes an increase in GK through muscarinic Me-receptors (Fig. ID, E) 33'34. Application of NE or stimulation of the locus coeruleus increases the excitability of both LGNd relay and nRt neurons35'36. Intracellular investigations have revealed that these facilitatory effects are due to an oq-adrenoceptor-coupled decrease in GK (Fig. 2B, C) 17. This decrease in GK has many effects, including the generation of a slow depolarization that brings Cerebral cortex

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application results in a slow depolarization due to a reduction in membrane potassium conductance. The slow depolarization shifts the neuron from the burst firing mode (pre; expanded for detail) to the single spike mode (post). The cell in (B) is a relay cell from the parataenial thalamic nucleus. Similar results were obtained in LGNd relay neurons. (C) NE causes the same response in nRt cells. The ionic action of NE on cortical and thalamic intemeurons is not known. (B and C are taken, w i t h permission, f r o m Ref. 17.) TINS, VoL 12, No. 6, 1989

217

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nRt n e u r o n s Fig. 3. Cellular mechanisms of spindle wave generation. (A) Cortical field potential recording during the occurrence of spindles (asterisks; filtered to illustrate these 7-14 Hz oscillations). (B, C) Intracellular recording from thalamocortical relay (B) and nRt (C) neurons during the generation of a spindle sequence. Thalamic relay cells undergo a series of rhythmic hyperpolarizations (presumably representing IPSPs), the rebound of which can generate a low-threshold Ca2+ spike (arrows). At the same time nRt neurons generate bursts of action potentials riding on a slow depolarization. (D) Simplified schematic diagram of anatomical interactions between nRt cells and thalamocortical relay neurons. The nRt neurons possess recurrent collaterals as well as dendrodendritic synapses (see Ref. 2). The nRt and other thalamic nuclei are reciprocally connected, although nRt cells are inhibitory and thalamocortical relay cells are excitatory. (A is taken, with permission, from Ref. 46; B and C from Ref. 47.) 218

the membrane potential closer to single spike firing threshold (Fig. 2B, post) and an increase in the input resistance and membrane length and time constants. Through these actions, NE facilitates neuronal responsiveness to excitatory and inhibitory synaptic inputs. Effects of ACh on LGNd neuronal response properties Local application of ACh results in a pronounced increase in responsiveness of LGNd relay neurons to visual stimuli 37. Stimulation of the brainstem cholinergic neurons projecting to the LGNd also causes an increase in relay cell responsiveness which is concomitant with a reduction in intrathalamic IPSPs 38. The effects of NE on LGNd receptive field responses have not yet been reported. However, stimulation of the locus coeruleus in the rat has been reported to increase the spontaneous firing rate of LGNd relay and nRt neurons, while decreasing the excitability of intrageniculate interneurons 35. Understanding the consequences of the actions of ACh and NE on thalamocortical processing is complicated by the fact that thalamic neurons possess more than one mode of action potential generation. For this reason, I now turn to the electrophysiological properties of thalamic neurons. Dual role of thalamus: rhythm generator and gate to cerebral cortex Numerous studies have implicated the thalamus in the generation and coordination of certain types of thalamocortical rhythm (e. g. spindle waves) (see Fig. 3; reviewed in Refs 2, 3). Indeed the state of the EEG is tightly coupled to the type of activity generated by thalamocortical relay neurons 2'3'3t. When thalamic neurons exhibit burst activity (Figs 3B, 4A, - 75 mV), the EEG is synchronized (e.g. Fig. 3A). In contrast, when these neurons display single spike activity (Fig. 4A, - 5 3 mV), the EEG is desynchronized. These two states of neuronal activity arise from the intrinsic electrophysiological properties of thalamic cells and intrathalamic connections, especially those between the GABAergic nRt and the specific relay and intralaminar nuclei (see Fig. 3) 2'31'39. Burst firing in thalamic neurons is due to the presence of a large, low threshold Ca 2+ current 2'39. The voltage-dependent properties of this current are such that when the membrane potential is more negative than - 6 5 mV, thalamic neurons can exhibit a low threshold Ca 2+ spike that generates a high frequency burst of fast Na+/K+-mediated action potentials (Fig. 4A, - 75 mV). However, if the neuron is tonically depolarized positive to - 6 5 mV, then the more traditional single spike, or transfer, mode of neuronal activity is generated (Fig. 4A, - 5 3 mV). These two modes of action potential generation have very different functional consequences for the transfer of synaptic inputs to the cerebral cortex. In the burst firing mode the output of the relay neuron is highly non-linear in relation to the input that triggers it, while in the single spike firing mode, input and output are much more linearly related 4°. The transfer of information through the thalamus to the cerebral cortex is accurate only when the thalamic neuron is in the transfer mode and not in the burst firing mode 41. The high correlation between the mode of action TINS, Vol. 12, No. 6, 1989

potential generation in single thalamic neurons and the state of the EEG in the animal gives rise to the exciting possibility that observations on thalamic neuronal membrane properties made in vitro can be directly related to behavioral observations. In this manner, the ability of neurotransmitters to change the firing mode of thalamic neurons from one state to the other is of particular interest, because it may underlie, in part, the animal's shift from a state of EEG synchronization (e.g. drowsiness, inattentiveness or slow-wave sleep) to one of EEG desynchronization [e.g. arousal, alertness or paradoxical (dream) sleep] (see Fig. 4B, C). C h o l i n e r g i c and noradrenergic c o n t r o l of t h a l a m i c firing mode

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The search for extrathalamic inputs responsible for the large changes in thalamic firing patterns during shifts in arousal have 1 2s~ centred around cholinergic and noradrenergic brainstem inputs 2']7'3z-34'38'42'43. In this way, both the ACh- and NE-induced c slow depolarizations of thalamic neurons are very effective at inhibiting the occurrence of burst S-sleep firing by depolarizing the neuron ÷ out of the voltage range in which ! ' I I i ; the low threshold Ca2+ current is 2 ,,~ m active 17'32. Since these depolarizP-sleep 100 ms ations bring the membrane potential closer to single spike firing Fig. 4. Firing properties of thalamic neurons and their alteration with shifts in sleep-wake cycle. threshold ( - 5 5 mV) they have the (A) At - 7 5 mV, thalamic cells respond to a depolarizing current pulse with a slow Ca 2+ spike additional effect of promoting the (arrow) that triggers a burst of three Na +-dependent action potentials. This type of cellular activity shift to the transfer mode of spike is known as burst firing. Depolarizing the cell to - 6 3 mV inhibits burst firing by inactivating the low activity (Fig. 2B). threshold Ca2+ current. The response of the cell is now entirely passive. Further depolarization to An additional mechanism by - 5 3 mV results in the current pulse generating a train of four action potentials. This latter pattern which the thalamus controls infor- of spike activity is known as the single spike or transfer mode of action potential generation. mation transfer to the cerebral Decreasing resting membrane potassium conductance (GK) is very effective in shifting the neuron cortex is through the use of local from the burst to the single spike firing mode. (B) Intracellular recording from an LGNd neuron GABAergic interneurons. In the during transition from slow-wave sleep (S-sleep) to paradoxical sleep (P-sleep) and vice versa. LGNd for instance, axons of retinal SPOL (sommeil phasique & ondes lentes) is an intermediary stage between S-sleep and P-sleep and ganglion cells form monosynaptic is characterized by ponto-geniculo-occipital (PGO) waves48. Slow-wave sleep is characterized by the presence of rhythmic burst discharges (expanded in C1) and a relatively hyperpolarized connections with the dendrites of membrane potential Paradoxical sleep is characterized by single spike activity (C2) and a relatively both thalamocortical relay cells and depolarized membrane potential Similar shifts in membrane potential are found in the transition local GABAergic interneurons from slow-wave sleep to waking. Depolarization of the membrane potential in paradoxical sleep (reviewed in Ref. 42). In the cat's • may represent the influence of increased activity of brainstem chofinergic neurons while the X-pathway, the dendrites of inter- depolarization upon waking may be due to a mixture of cholinergic and noradrenergic influence. neurons in turn establish synaptic The role of corticothalamic projections in these shifts is not yet known. (B and C are taken, with contacts with dendrites of the relay permission, from Ref. 48.) cell, thereby forming a tri-synaptic loop42 (Fig. 1, inset). As a result of this anatomical although controversial, has long been a major arrangement, the duration and amplitude of post- mechanism thought to explain the well-known facilisynaptic EPSPs evoked by retinal inputs are under tatory effects of brainstem stimulation on the transfer direct control of intrageniculate interneurons. Small of phasic EPSPs through the thalamus 43. In favor of reductions in the excitability of local GABAergic the cholinergic disinhibition hypothesis are two main interneurons may therefore facilitate the postsynaptic findings. (1) Electrical stimulation in the brainstem potentials generated by visual stimuli. results in inhibition, through muscarinic receptors, of Cholinergic reduction of intrathalamic inhibition, both intrageniculate- and perigeniculate (PGN)-nRt TINS, Vol. 12, No. 6, 1989

219

neurons, and reduces IPSPs in relay cells:~8'43'44. (2) Application of ACh to morphologically and electrophysiologically identified intrageniculate and nRt interneurons results in inhibition of these cells through an increase in membrane potassium conductance (Fig. 1D, E) ~'34. However, in considering the cholinergic disinhibition hypothesis, one must take into account two other findings. (3) Local iontophoretic application of ACh in vivo has been reported to block only inhibition arising from distal portions of the receptive field and not that which underlies center-surround antagonism 45. (4) During naturally occurring arousal, there appears to be an increase, and not a decrease, in the effectiveness of intrathalamic inhibitory mechanisms 4~. Each of these findings has its potential flaws: (1) synchronous stimulation of brainstem cholinergic inputs is likely to be a poor substitute for how these neurons fire during naturally occurring arousal; (2) intracellular recordings in interneurons are probably representative of cholinergic actions in the cell body and proximal dendrites which may be different from cholinergic action in synaptic glomeruli; (3) local iontophoretic application of ACh in vivo is unlikely to reach all of the interneurons involved in intrathalamic inhibitory processes, and therefore will underestimate the influence of cholinergic disinhibition; and finally, (4) the reduced effectiveness of inhibitory processes during EEG synchronization may not represent a true reduction in IPSP amplitude, but rather an inability of these IPSPs to suppress completely the large and powerful currents (e.g. low threshold Ca2+ current) that dominate the discharge pattern of thalamic relay cells in this behavioral state (e.g. see Fig. 1C in Ref. 41). The vast majority of studies indicate that the ascending cholinergic system is capable of reducing at least some forms of intrathalamic inhibition 31-34'3s'43-45. his effect of ACh may decrease both feed-forward (intrageniculate) and feedback (nRt) inhibition and thereby increase the amplitude and duration of incoming EPSPs. In this manner, cholinergic inputs may balance (i. e. reduce without blocking) intrathalamic inhibitory processes so that thalamocortical processing occurs in an accurate and efficient manner without destro~ng the visual processes that rely upon intrathalamic GABAergic inhibition. Together with the direct depolarization of relay neurons by ACh and NE, these actions will potently inhibit intrathalamic rhythm generation and promote the accurate and faithful processing of information. Thalamocortical activation: a possible

scenario Transition from synchronized, rhythmic thalamocortical activity during slow-wave sleep to desynchronized, high-frequency activity during arousal and attentiveness is believed to be associated with a general increase in the firing rate of cholinergic, noradrenergic, serotonergic, and histaminergic neurons 7-12. (Paradoxical sleep is an exception to this generalization since in this state, both the noradrenergic and serotonergic neurons undergo a marked decrease in discharge rate, whereas that of brainstem cholinergic neurons increases.) The increase in release of these agents will have numerous and complex actions on forebrain neurons (e.g. see Refs 220

13-38). However, one important and common response of cerebral cortical pyramidal cells to all of these agents is a marked reduction in spike frequency adaptation due, in part, to block of IAnP13-16'18 (McCormick, D. A. and Williamson, A., unpublished observations). Consequently, the response of these cells to trains of EPSPs, such as those that occur during stimulation of receptive fields, is enhanced, while other inputs (e.g. IPSPs) remain relatively unchanged. In the thalamus, the increased release of ACh and NE will cause a slow depolarization of thalamocortical relay cells by blocking a resting potassium current. This slow depolarization inactivates the low threshold Ca2÷ spike and therefore inhibits rhythmic burst discharges. Furthermore, the depolarization towards single spike firing threshold, the increase in specific membrane resistance, as well as the reduction (by ACh) of over-powerful (i.e. shunting) inhibitory processes, will all increase the likelihood that phasic EPSPs will trigger action potentials in a one-to-one manner 4°'41 and thus result in the faithful transfer of incoming spike trains to the cerebral cortex. Although a great many questions remain to be solved, it is clear that the ascending neurotransmitter systems possess potent and versatile mechanisms through which they can control the excitability and processing state of neurons throughout the forebrain. These systems may act to adjust and 'tune' the status of cells in the brain so that the demands placed upon them are met in as efficient and accurate a manner as possible. Selected references 1 Brazier, M. A. B. (1980) in The Reticular Formation Revisited (Hobson, J. A. and Brazier, M. A. B., eds), Raven Press 2 Steriade, M. and Deschenes, M. (1984) Brain Res. Rev. 8, 1--63 3 Hobson, J. A. and Steriade, M. (1986) in Handbook of Physiology (Sect. 1: The Nervous System; Pt IV: Intrinsic Regulatory Systems of the Brain), pp. 401-823, Williams & Wilkins and the American Physiological Society 4 Moruzzi, G. and Magoun, H. W. (1949) Electroencephalogr. Clin. Neurophysiol. 1,455-473 5 Mesulam, M. M. (1988) in Neurotransmitters and Cortical Function (Avoli, M., Reader, T. A., Dykes, R, W. and Gloor, P., eds), pp. 237-260, Plenum Press 6 Lindvall, O. and Bj&klund, A. (1984) in Monoamine Innervation of Cerebral Cortex (Descarries, I., Reader, T. A. and Jasper, H. H., eds), pp. 9-40, Alan R. Liss 7 Foote, S. L. and Morrison, J. H. (1987) Annu. Rev. Neurosci. 10, 67-95 8 Pollard, H. and Schwartz, J-C. (1987) Trends NeuroscL 10, 86-89 9 Aston-Jones, G. and Bloom, F. E. (1981) J. Neurosci. 1, 876-886 10 Trulson, M. E. and Jacobs, B. L. (1979) Brain Res. 176, 135-150 11 Vanni-Mercier, G., Sakai, K. and Jouvet, M. (1984) C. R. Acad. Sci. Ser. III 298, 195-200 12 Lamour, Y., Dutar, P., Rascol, O. and Jobert, A. (1986) Brain Res. 362, 122-131 13 Madison, D. V. and Nicoll, R. A. (1982) Nature299, 636-638 14 Haas, H. L. and Konnerth, A. (1983) Nature 302,432-434 15 McCormick, D. A. and Prince, D. A. (1986) J. Physiol. (London) 375, 169-194 16 Colino, A. and Halliwell, J. V. (1987) Nature 328, 73-77 17 McCormick, D. A. and Prince, D. A. (1988) J. Neurophysiol. 59, 978-996 18 Foehring, R. C., Schwindt, P. C. and Crill, W. E. (1989) J. Neurophysiol. 61,245-256 19 Madison, D. V., Lancaster, B. and Nicoll, R. A. (1987) J. Neurosci. 7, 733-741 20 Halliwell, J. V. (1986) Neurosci. Left. 67, 1-6

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21 Brown, D. A. and Adams, P. R. (1980) Nature 283,673-676 22 Pennefather, P., Lancaster, B., Adams, P. R. and Nicoll, R. A. (1985) Proc. Natl Acad. Sci. USA 82, 3040-3044 23 Madison, D. V. and Nicoll, R. A. (1984) J. Physiol. (London) 354, 319-331 24 Siggins, G. R. and Gruol, D. L. (1986) in Handbook of

Physiology (Sect. 1: The Nervous System; Pt IV Intrinsic Regulatory Systems of the Brain), pp, 1-114, Williams & Wilkins and the American Physiological Society 25 Armstrong-James, M. and Fox, K. (1983)J. PhysioL (London) 335, 427--447 26 Sillito, A. M. and Kemp, J. A. (1983) Brain Res. 289, 143-155 27 Lamour, Y., Dutar, P., Jobert, A. and Dykes, R. W. (1988) J. Neurophysiol. 60, 725-750 28 Waterhouse, B. D. et al. (1988) Brain Res. Bull. 21,425-432 29 De Lima, A. D. and Singer, W. (1987) J. Comp. Neurol. 259, 92-121 30 De Lima, A. D., Montero, V. M. and Singer, W. (1985) Exp. Brain Res. 59, 206-212 31 Steriade, M. and Llin,~s, R. R. (1988) Physiol. Rev. 68, 649-742 32 McCormick, D. A. and Prince, D. A. (1987) J. PhysioL (London) 392, 147-165 33 McCormick, D. A. and Prince, D. A. (1986) Nature 319, 402--405 34 McCormick, D. A. and Pape, H-C. (1988) Nature 334, 246-248

35 Kayama, Y. (1985) Vision Res. 25, 339-347 36 Rogawski, M. and Aghajanian, G. K. (1980) Nature 287, 731-734 37 Sillito, A. M., Kemp, J, A. and Berardi, N. (1983) Brain Res. 280, 299-307 38 Francesconi, W., Muller, C. M. and Singer, W. (1988) J. Neurophysiol. 59, 1690-1718 39 Jahnsen, H. and Llin&s, R. R. (1984)J. Physiol. (London)349, 205-247 40 Feeser, H. R. and McCormick, D. A. (1988) Soc. Neurosci. Abstr. 14, 277 41 Livingstone, M. S. and Hubel, D. H. (1981) Nature 291, 554-561 42 Sherman, S. M. and Koch, C. (1986) Exp. Brain Res. 63, 1-20 43 Singer, W. (1977) Physiol. Rev. 57, 386-420 44 Ahlsen, G., Lindstrom, S. and Lo, F. S. (1984) J. Physiol. (London) 347, 593-609 45 Eysel, U. T., Pape, H-C. and Van Schayck, R. (1986) J. Physiol. (London) 370, 233-254 46 Steriade, M., Domich, L., Oakson, G. and Deschenes, M. (1987) J. Neurophysiol. 57, 260-273 47 Steriade, M. and Deschenes, M. (1988) in Cellular Thalamic Mechanisms (Bentivoglio, M. and Spreafico, R., eds), pp. 37-62, Elsevier 48 Hirsch, J. C., Fourment, A. and Marc, M. E. (1983) Brain Res. 259, 308--312

Acknowledgements I thank Mircea Steriade, Martin Desch#nes,June Hirsch and their colleagues for permission to reprint portions of their data in this article. I am indebted to David Princeand HansChristian Papefor their collaboration on portions of these projects. Supported by JacobsJavits Centerin Neuroscience, NINCDS, and a fellowship from the Klingenstien Foundation.

Unravelingprion diseasesthrough moleculargenetics David Westaway,

G e o r g e A. C a r l s o n a n d S t a n l e y B. P r u s i n e r

Prions are transmissible pathogens that cause degenerative diseases in humans and animals. Unique attributes of prion diseases include infectious, sporadic and genetic manifestations, as well as progression to death, all in the absence of a detectable immune response. Prions are resistant to chemical procedures that modify or destroy nucleic acids and are composed largely of a protein, designated PrP so. Molecular cloning of a co~,nate cDNA established a cellular host origin for PrP °c protein and a convergence with the genetics of host susceptibility. The murine PrP gene is linked to the Prn-i gene which determines incubation times in experimental scrapie. Mice with long incubation times have unusual PrP alleles encoding phenylalanine and valine at codons 108 and 189. Moreover, the ataxic form of Gerstmann-Strdussler syndrome (a rare human neurodegenerative disorder) has been defined as an autosomal dominant disorder with a PrP mis-sense mutation at codon 102 linked to the predisposition locus. These studies argue that amino acid substitutions in 'PrP' genes may modulate initiation and development of prion diseases.

scrapie infectivity was filterable, the aetiological agent was highly resistant to a variety of procedures that destroy or modify nucleic acids 1. Instead, infectivity was associated with a proteinaceous component, subsequently identified as the scrapie prion protein, PrP so. Prion diseases are distinguished also from conventional infectious disease by their unusual biological features. Scrapie, CJD and GSS proceed in the apparent absence of a detectable immune response 2. When the familial versions of the human priori diseases are excluded, these diseases occur in isolation ('sporadically') or iatrogenically. Given the experimental transmissibility of the diseases, it was reasonable to attribute sporadic disease to cryptic vertical or lateral spread from an affected individual or from an animal reservoir. However, epidemiologists have failed to identify such vectorial 3 spread and studies of maternal transmission of kuru and CJD prions were similarly negative 4'5. Tripartite manifestation - infectious, familial and sporadic - may be an intrinsic feature of all prion diseases in nature. Molecular genetic studies summarized here reveal that PrP sc molecules are encoded by the host, and Seven prion diseases of animals and humans are now that this genetic origin may contribute to the unique recognized. All are degenerative neurological dis- biological attributes of prions. orders that can be transmitted by inoculation. The pathological features of prion diseases include neur- Prions contain PrP sc PrP27-30, the protease-resistant core of PrP sc, onal vacuolafion, astrocytic gliosis and deposition of amyloid plaques. In animals, prions are the cause of was discovered by enriching brain fractions for scrapie scrapie of sheep and goats, of transmissible mink infectivity6'7. Development of a more rapid and encephalopathy, of chronic wasting disease of mule economical bioassay8 greatly facilitated purification of deer and elk and of bovine spongiform encephal- the hamster scrapie agent 7. PrP27-30 migrates opathy. In humans, prions cause kuru, Creutzfeldt- during SDS-PAGE as a broad band with an apparent Jakob disease (CJD) and Gerstmann-Str~iussler syn- molecular weight of 27 000-30 000. drome (GSS). Correspondence should be addressed to David Westaway: The unusual physiochemical nature of the prions Department of Neurology, HSE-781, University of Cafifornia, San was apparent more than two decades ago. Although Francisco, CA 94143-0518, USA. TINS, Vol. 12, No. 6, 1989

© 1989.ElsevierSciencePublishersLtd,(UK) 0166-2236/89/$02.00

David Westawayand Stanley B. Prusinerare at the Departments of Neurology, Biochemistryand Biophysics, University of California, San Francisco, CA 94143, USA and GeorgeA. Carlsonis at the McLaughlin Research Institute, 6reat Falls, MT 5940f, USA.

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