A quantitative microiontophoretic analysis of the responses of central neurones to noradrenaline: interactions with cobalt, manganese, verapamil and dichloroisoprenaline

Probing presynaptic regulation of extracellular dopamine with iontophoresis

Iontophoresis allows for localized drug ejections directly into brain regions of interest driven by the application of current. Our lab has previously adapted a method to quantitatively monitor iontophoretic ejections. Here those principles have been applied in vivo to modulate electrically evoked release of dopamine in anesthetized rats. A neutral, electroactive marker molecule that is ejected purely by electroosmotic flow (EOF) was used to monitor indirectly the ejection of electroinactive dopaminergic drugs (raclopride, quinpirole, and nomifensine). Electrode placements were marked with an iontophoretically ejected dye, pontamine sky blue. We show that EOF marker molecules, acetaminophen (AP) and 2-(4-nitrophenoxy) ethanol, have no effect on electrically evoked dopamine release in the striatum or the sensitivity of electrode. Additionally, we establish that a short, 30 second ejection of raclopride, quinpirole, or nomifensine with iontophoresis is sufficient to affect autoreceptor regulation and the re-uptake of dopamine. These effects vary in lifetime, indicating that this technique can be used to study receptor kinetics.

Layer-specific noradrenergic modulation of inhibition in cortical layer II/III

Norepinephrine (NE) is released in the neocortex after activation of the locus coeruleus of the brain stem in response to novel, salient, or fight-or-flight stimuli. The role of adrenergic modulation in sensory cortices is not completely understood. We investigated the possibility that NE modifies the balance of inhibition acting on 2 different γ-aminobutyric acid (GABA)ergic pathways. Using patch-clamp recordings, we found that the application of NE induces an α(1) adrenergic receptor-mediated decrease of the amplitude of inhibitory postsynaptic currents (IPSCs) evoked by stimulation of layer I (LI-eIPSCs) and a β and α(2) receptor-mediated increase in the amplitude of IPSCs evoked by stimulation of layer II/III (LII/III-eIPSCs). Analysis of minimal stimulation IPSCs, IPSC kinetics, and sensitivity to the GABA(A) receptor subunit-selective enhancer zolpidem corroborated the functional difference between LI- and LII/III-eIPSCs, suggestive of a distal versus somatic origin of LI- and LII/III-eIPSCs, respectively. These findings suggest that NE shifts the balance between distal and somatic inhibition to the advantage of the latter. We speculate that such shift modifies the balance of sensory-specific and emotional information in the integration of neural input to the upper layers of the auditory cortex.

Antioxidant protection of cerebellar beta-adrenergic receptor function in aged F344 rats

We examined whether a 2-week treatment with the spin-trapping agent MDL 101.002 (MDL) or a diet supplemented with vitamin E (Vit E) would alleviate age-related deficits in cerebellar noradrenergic function of male 18-20 month old F344 rats compared to age-matched controls. Cerebellar beta-adrenergic receptor function was assessed using extracellular recordings of Purkinje cells during iontophoresis of GABA and isoproterenol (ISO). Noradrenergic receptor function of MDL and Vit E-treated rats was similar to young rats whereas for non-treated rats it was typical of that previously recorded in aged rats. Thus, treatment with MDL or Vit E reverses age-related deficits in cerebellar noradrenergic receptor function.

Electrophysiological evidence that noradrenergic neurons of the rat locus coeruleus are tonically inhibited by GABA during sleep

It is well known that noradrenergic locus coeruleus (LC) neurons decrease their activity during slow wave sleep (SWS) and are virtually quiescent during paradoxical sleep (PS). It has been proposed that a GABAergic input could be directly responsible for this sleep-dependent neuronal inactivation. To test this hypothesis, we used a new method combining polygraphic recordings, microiontophoresis and single-unit extracellular recordings in unanaesthetized head-restrained rats. We found that iontophoretic application of bicuculline, a specific GABA(A)-receptor antagonist, during PS and SWS restore a tonic firing in the LC noradrenergic neurons. We further observed that the application of bicuculline during wakefulness (W) induced an increase of the discharge rate. Of particular importance for the interpretation of these results, using the microdialysis technique, Nitz and Siegel (Neuroscience, 1997; 78: 795) recently found an increase of the GABA release in the cat LC during SWS and PS as compared with waking values. Based on these and our results, we therefore propose that during W, the LC cells are under a GABAergic inhibitory tone which progressively increases at the entrance and during SWS and PS and is responsible for the inactivation of these neurons during these states.

Beta-adrenergic modulation of GABAergic inhibition in the deep cerebellar nuclei of F344 rats

The presence of norepinephrine (NE) and NE activated cells, in the deep cerebellar nuclei (DCN) of male F344 rats, was investigated using immunohistochemistry and electrophysiology, during iontophoresis of the beta-adrenergic agonist isoproterenol (ISO). During extracellular electrophysiology, GABA was iontophoretically applied to the cell and ISO was then co-applied in an attempt to modulate the GABAergic inhibition of cell firing in the DCN. Immunohistochemistry was used to detect tyrosine hydroxylase (TH) positive fibers in the DCN. Isoproterenol modulated GABAergic inhibition in 51% of the DCN cells recorded from. In addition, TH-positive fibers that appeared to make contact with DCN cells were found. Therefore, this study demonstrated that functional NE receptors exist in the DCN and NE appears to be present in fibers therein.

The role of GABAA receptors in the subsensitivity of Purkinje neurons to GABA in genetic epilepsy prone rats

The GABA receptor subtype mediating responses of cerebellar Purkinje neurons to the neurotransmitter was evaluated and compared in GEPR-9 vs. nonepileptic, genetic control GEPR-NE rats. Quantitative analysis of responses to microiontophoretically applied GABA, muscimol and baclofen indicated that the inhibitory action of GABA on cerebellar Purkinje neurons was mediated by GABAA receptors since muscimol produced responses similar to those of GABA and baclofen was without substantial electrophysiological action. In addition, Purkinje neurons in GEPR-9 animals showed a similar reduced sensitivity to both GABA and muscimol. Radioligand binding studies using the GABAA receptor selective ligand, [3H]muscimol, and the benzodiazepine receptor selective ligand, [3H]flunitrazepam, were conducted on cerebellar and cortical homogenates from GEPR 9, GEPR-NE and Sprague-Dawley rats. No differences in the Kd or Bmax for these ligands among the three groups studied were observed. The lack of significant changes in the Kd and Bmax for these two ligands in the cerebellum suggests that the mechanism for the observed subsensitivity to GABA in the GEPR 9 rat lies beyond the level of the receptor, perhaps at the signal transduction process for GABA mediated inhibitory responses.

Effects of dietary restriction on motor learning and cerebellar noradrenergic dysfunction in aged F344 rats

Fisher 344 rats were fed either ad libitum or with a diet containing a 40% reduction of calories beginning at 4 months of age. At 14 months and 22 months male rats were tested for their ability to learn a complex motor skill. At both ages the diet restricted rats reached criterion of performing 10 successful crosses in 10 min at an earlier time than ad libitum fed controls. At 22 months of age the diet restricted rats showed improved acquisition of running times for the task. Male rats at 14 and 22 months and female rats at 24 months were examined electrophysiologically for the ability of isoproterenol to augment the action of GABA in the cerebellum when both substances were applied iontophoretically from an extracellular multibarreled glass electrode. In all 3 age and sex groups there was an improvement in the beta-adrenergic receptor modulation of GABA responses in the dietary restricted vs. ad libitum rats. However, no difference was observed between dietary restricted and ad libitum rats when the number and affinity of cerebellar beta-adrenergic receptors was assessed with 125I-iodopindolol binding. Overall, there was a significant improvement in cerebellar noradrenergic function in the dietary restricted rats and this was accompanied by an improvement in motor learning.

Aging and motor learning: a possible role for norepinephrine in cerebellar plasticity

Norepinephrine is known to act as a neuromodulator in the cerebellar cortex because it can increase the effect of neurotransmitters such as GABA. This neuromodulatory effect of NE is a possible substrate for an effect of NE on cerebellar plasticity. Cerebellar plasticity can be examined by studying the learning of motor skills. A rod walking paradigm is used in our laboratory for such investigations. Learning of this rod walking task is impaired in rats that are depleted of central stores of NE and in rats that have received the beta-adrenergic antagonist propranolol. In addition, in aged rats there is a correlation between the loss of beta-adrenergic receptor mediated neuromodulatory actions of NE in the cerebellum with a decreased ability to learn the rod walking task. Taken together this information supports a role for NE in cerebellar plasticity and suggest that the beta-adrenergic receptor is important for this plasticity.

The effects of chronic treatment with N-tert-butyl-alpha-phenylnitrone on cerebellar noradrenergic receptor function in aged F344 rats

We examined if a 2 week treatment with the spin trapping agent N-tert-butyl-alpha-phenylnitrone (PBN) would alleviate age-related deficits in cerebellar noradrenergic function of male 21-22 month old F344 rats compared to age-matched controls. Noradrenergic receptor function of PBN-treated rats was similar to young rats whereas for non-treated rats it was typical of that previously recorded in aged rats. Thus, treatment with PBN may reverse age-related deficits in cerebellar noradrenergic receptor function.

Motor learning deficits in aged rats are correlated with loss of cerebellar noradrenergic function

We have demonstrated that aged rats show impairments in learning patterned motor movements. Similar behavioral impairment is observed in rats with noradrenergic lesions. Norepinephrine is known to act as a neuromodulator in the cerebellar cortex because it can augment the action of GABA and other neurotransmitters. This effect of NE to augment the signal to noise ratio of GABAergic inputs to cerebellar Purkinje neurons is a possible substrate for NE's effect on motor learning. Aged rats demonstrate deficits in the modulatory actions of NE to augment GABAergic inhibitions when both substances are locally applied onto cerebellar Purkinje neurons. In this report, we examined how motor learning and cerebellar noradrenergic function varied in individual young and 20-month-old Fischer 344 rats. There was a significant correlation between the loss of the neuromodulatory actions of norepinephrine (NE) in the cerebellar cortex and the rate of learning a novel motor task in individual rats. This report thus demonstrates for the first time a correlation between age-related impairments in motor plasticity and specific neurophysiological deficits in cerebellar Purkinje neurons in individual animals.

In vivo electrochemical measurements and electrophysiological studies of rat striatum following neonatal 6-hydroxydopamine treatment

The effects of neonatal treatment (one day after birth) with the neurotoxin, 6-hydroxydopamine (75 micrograms/10 microliters intracisternal), were studied in the striatum of normal adult and treated Sprague-Dawley rats. Measurements of monoamine levels in the dorsal striatum and nucleus accumbens, by high-performance liquid chromatography coupled with electrochemical detection, showed that neonatal 6-hydroxydopamine treatment produced a permanent and massive destruction of striatal dopamine. The effects were more pronounced in the dorsal striatum than in the nucleus accumbens. In addition, serotonin levels were elevated in the rat striatum as a consequence of the neonatal treatment. Rapid chronoamperometric recordings of K(+)-evoked monoamine overflow using Nafion-coated recording electrodes were investigated in both the dorsal and ventral striatum of control and neonatally lesioned rats. The potassium-evoked responses recorded from the dorsal striatum of the 6-hydroxydopamine-treated rats were significantly reduced in amplitude as compared to controls. In addition, the reduction/oxidation current ratios of the responses were more serotonin-like, in contrast to the dopamine-like current ratios measured in the striatum of untreated animals. In ventral striatum, the amplitudes of the K(+)-evoked responses were not significantly reduced versus control. However, the K(+)-evoked signals were more serotonin-like in their electrochemical characteristics as compared to controls. In addition to the release studies, extracellular single-unit electrophysiological recordings were performed in normal and neonatally 6-hydroxydopamine-treated rats. The spontaneous discharge rate of striatal neurons in the neonatally 6-hydroxydopamine-treated rats was similar to that of control rats. This is in contrast to dopamine lesions in adult animals, where a marked elevation of the discharge rate is observed. Local applications of dopamine and serotonin into the striatum of neonatally 6-hydroxydopamine-treated rats elicited excitations of striatal cells rather than the normal inhibitory effects seen in control animals. Taken together, these data suggest that loss of striatal dopamine terminals at birth leads to both pre- and postsynaptic alterations in monoamine pathways.

Immunohistochemical demonstration of regionally selective projections from locus coeruleus to the vestibular nuclei in rats

This study describes a regionally selective projection of tyrosine hydroxylase and dopamine beta-hydroxylase-immunoreactive fibers from locus coeruleus (LC) and the A4 region of nucleus subcoeruleus to the vestibular nuclear complex in Long-Evans and Sprague-Dawley rats. These fibers travel in two distinct pathways. A lateral descending noradrenergic bundle provides input from LC to the superior vestibular nucleus (SVN), the cochlear nuclei, and the cerebellar cortex. A medial descending noradrenergic bundle provides input to the lateral vestibular nucleus (LVN), medial vestibular nucleus (MVN), and the inferior vestibular nucleus (IVN) before continuing on to the cochlear and cerebellar nuclei. The terminal plexus of these fibers varies markedly across these vestibular nuclear regions. Immunoreactive axons form a dense plexus around somata and proximal dendrites of Deiters' neurons in dorsal LVN. The axon plexus is less dense in SVN and ventral LVN, and relatively sparse in MVN and IVN. This regional selectivity of noradrenergic innervation suggests that central adrenergic systems may selectively modulate vestibulospinal reflexes at the level of the vestibular nuclear complex.

Noradrenergic agents into the cerebellar anterior vermis modify the gain of vestibulospinal reflexes in the cat

The noradrenergic (NA) afferent projection to the cerebellar cortex, which originates mainly from the locus coeruleus (LC), may act on the target neurons by utilizing both alpha- and beta-adrenoceptors. Experiments performed in decerebrate cats have shown that unilateral injection into the vermal cortex of the cerebellar anterior lobe of 0.25 microliter of the alpha 1-adrenergic agonist metoxamine or the alpha 2-agonist clonidine (at 2-8 micrograms/microliters of saline) as well as of the non-selective beta-agonist isoproterenol (at 8-16 micrograms/microliters) decreased the postural activity in the ipsilateral forelimb, while the extensor tonus either remained unmodified or slightly increased on the contralateral side. The same agents also increased the gain of the vestibulospinal (VS) reflexes elicited by recording the multiunit EMG responses of the ipsilateral and the contralateral triceps brachii to roll tilt of the animal (at 0.15 Hz, +/- 10 degrees), leading to sinusoidal stimulation of labyrinth receptors. The crossed effects were more prominent for the alpha 2- than for the alpha 1- and beta-agonists. Only slight changes in the phase angle of the responses were observed. The effects described above appeared 5-10 min after the injection, reached the peak values after 15-30 min and disappeared within 2 h. The effective area was located within the third and/or the fourth folium of the culmen rostral to the fissura prima, 1.4-1.8 mm lateral to the midline. This area corresponded to zone B of the cerebellar cortex, which projects to the ipsilateral lateral vestibular nucleus (LVN), on which it exerts a prominent inhibitory influence. In fact, monopolar stimulation of this area with three negative pulses (at 300/sec) performed prior to the local injection inhibited the spontaneous EMG activity of the ipsilateral triceps brachii. The effects described above were dose-dependent; injection of an equal volume of saline was ineffective. All changes in posture and reflexes elicited by metoxamine or clonidine were impaired by previous injection into the same corticocerebellar area of the corresponding alpha 1- or alpha 2-adrenergic antagonist prazosin or yohimbine, respectively (0.25 microliters at 8-16 micrograms/microliters). However, cross-interactions between alpha 1- and alpha 2-adrenergic agonists and antagonists were also observed. In fact, injection of the alpha 2-adrenergic antagonist yohimbine prevented the occurrence of all the metoxamine effects, while administration of the alpha 1-adrenergic antagonist prazosin prevented the occurrence of the ipsilateral, but not of the contralateral effects induced by clonidine injection.(ABSTRACT TRUNCATED AT 400 WORDS)

Age-related subsensitivity of cerebellar Purkinje neurons to locally applied beta 1-selective adrenergic agonist

Previous electrophysiological studies in aged rats have revealed a number of deficits in noradrenergic neurotransmission in the central nervous system. Such deficits include subsensitivity to the depressant effects of norepinephrine on cerebellar Purkinje neurons, which has been attributed specifically to altered beta adrenergic receptor-mediated processes. The objective of this study was to determine which beta adrenergic receptor subtype, beta 1 or beta 2, is responsible for this age-related subsensitivity. The effects of beta 1 and beta 2 agonists on spontaneous activity of Purkinje neurons was first examined in young rats and the selectivity of these agents was validated using selective beta 1 and beta 2 antagonists. The effects of the selective beta 1 and beta 2 agonists were then compared in young (3-month-old) and aged (18- and 26-month-old) Fischer 344 rats. These agents were applied to Purkinje neurons by pressure microejection from multibarreled micropipettes and the change in neuronal action potential discharge rate was recorded. Both dobutamine, a beta 1-selective agonist, and zinterol, a beta 2-selective agonist, induced dose-dependent inhibitions of Purkinje cell firing rate. Dobutamine-induced inhibitions were blocked by the selective beta 1 antagonist, ICI 89406 and not by the beta 2-selective antagonist, ICI 118551; conversely, zinterol-induced inhibitions were not blocked by ICI 89406 but were blocked by the presence of ICI 118551. Purkinje neurons of both groups of aged rats were significantly less sensitive to locally applied dobutamine than Purkinje cells of young rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic modulation of cholinergic responses in rat medial prefrontal cortex: an electrophysiological study

The neuromodulatory action of dopamine (DA) on acetylcholine (ACh)-evoked responses of prefrontal cortex (PFC) neurones were investigated electrophysiologically in rats anaesthetised with a combination of urethane and ketamine. Iontophoretic application of ACh-excited prefrontal cortex neurones. Concurrent application of DA (5-15 nA) resulted in complex changes in the ACh-evoked responses: (1) DA depressed spontaneous background discharges (designated as noise) proportionally more than the ACh-evoked discharges (designated as input signals), thus yielding an enhanced signal/noise ratio. This increase in signal/noise ratio by dopamine was reversed by iontophoretic application of the Da D2 antagonist sulpiride (20-50 nA). Nevertheless, iontophoretic application of D2 agonist quinpirole (5-35 nA) enhanced the ACh-evoked response, but was accompanied by some increase in spontaneous discharge, thus yielding no change in the signal/noise ratio. (2) DA also increased the signal/noise ratio by inducing a net increase of the ACh-evoked response but simultaneously suppressed the spontaneous activity of PFC neurones. This effect was more prominent following blockade of D1 receptors by SCH23390 (6 mg/kg, i.p.), suggesting that D1 receptors may normally inhibit D2 receptor function in the PFC. In addition, endogenous DA in the PFC did not play a significant part in modifying the ACh-evoked responses since the modulation of ACh-evoked response by DA or its D1 and D2 agonists was similar in both saline control and alpha-methyl-p-tyrosine-pretreated rats. (3) When ejected with larger iontophoretic current (16-35 nA), DA suppressed both the ACh-evoked and spontaneous discharge and this effect was mimicked by D1 agonist SKF38393 (5-15 nA). Taken together, these results suggest that complex dopaminergic modulation of the cholinergic responses of prefrontal cortex neurones are mediated by D1 and D2 receptors. This DA action may have a functional role in the cognitive-integrative processes occurring in the prefrontal cortex.

Nicotine-induced inhibition of cerebellar Purkinje neurons: specific actions of nicotine and selective blockade by mecamylamine

The specificity and pharmacological characteristics of the effects of local administration of nicotine on cerebellar Purkinje cells in the rat were examined electrophysiologically. Local application of nicotine, whether by pressure-ejection or by iontophoresis, depressed the spontaneous discharge of Purkinje neurons in a reversible and dose-dependent manner. This action could not be mimicked by local application of vehicle alone. The inhibitory effects of (-)-nicotine were several-fold more potent than that of the (+)-enantiomer. Systemic administration of the ganglion blocker mecamylamine reliably and reproducibly antagonized the nicotine-induced inhibitions of Purkinje cells whereas nicotine-induced excitation of interneurons was not altered. Local pressure-ejection of mecamylamine also antagonized the inhibitory actions of nicotine, administered by iontophoresis. Since the central effects of nicotine on behavior are stereospecific and sensitive to mecamylamine, the data in this study further support the hypothesis that the actions of nicotine on Purkinje neurons are mediated by ganglionic-like receptors. These findings also suggest that the Purkinje cell may serve as a good cellular model for studies on central pharmacology of nicotine.

Kappa-bungarotoxin blockade of nicotine electrophysiological actions in cerebellar Purkinje neurons

The agonistic actions of nicotine in cerebellum were selectively blocked by kappa-bungarotoxin depending on the cell type studied. Nicotine-induced Purkinje cell inhibitions were antagonized by the simultaneous application of this toxin. In contrast, nicotine-induced cerebellar interneuron excitations were unaltered. These findings suggest that kappa-bungarotoxin may be used as a selective pharmacological tool for the study of nicotine actions which are dependent on ganglionic-like receptors, which have been associated with Purkinje cells in previous studies.

Electrophysiological effects of locally applied noradrenergic agents at cerebellar Purkinje neurons: receptor specificity

We have investigated the receptor subtype(s) mediating the noradrenergic inhibition of cerebellar Purkinje cell spontaneous firing rate using local application of specific agonists and antagonists, in situ, via pressure microejection. Extracellular action potentials were recorded from Purkinje neurons in anesthetized Fischer 344 rats. Timolol, a beta-receptor antagonist, did not affect norepinephrine (NE)-induced inhibition in 9 of 12 cells studied. Phentolamine, an alpha-receptor antagonist, blocked the effect of NE in 8 of 11 cells. To further determine the subtype of alpha-receptor involved, the effects of the alpha 1-antagonist prazosin and alpha 2-antagonists idazoxan and yohimbine were examined. While prazosin had no effect on NE-mediated inhibition, both idazoxan and yohimbine blocked NE effects. Idazoxan was also successful in blocking phencyclidine (PCP), an indirect noradrenergic agonist. The inhibitory action of NE upon Purkinje cell firing rate was mimicked by the selective alpha 2-agonist clonidine; this action of clonidine was blocked by idazoxan but not by timolol or prazosin. In addition, the alpha 1-adrenergic agonist phenylephrine and the beta-adrenergic agonist isoproterenol inhibited Purkinje cell firing rate. Phenylephrine effects were blocked by prazosin but not by timolol or idazoxan. Isoproterenol-induced inhibition was blocked by timolol but not phentolamine. Taken together, these studies suggest that both alpha- and beta-receptors alter Purkinje cell firing rate; the depressant action of locally applied NE, however, seems to be mediated primarily via an alpha 2-adrenergic receptor.

Electrophysiological effects of norepinephrine on Purkinje neurons in intraocular cerebellar grafts: alpha- vs beta-specificity

The present study investigates the receptor specificity of the electrophysiological effects of norepinephrine (NE) on cerebellar Purkinje neurons. Intraocular cerebellar grafts were utilized to allow both superfusion and local administration of selective adrenergic agonists and antagonists. Fetal cerebellar anlagen (E13-15) were homologously transplanted to the anterior chamber of the eye of adult recipient rats and allowed to mature in the eye for at least 5 weeks. Spontaneous activity of Purkinje neurons was recorded extracellularly in the intraocular grafts. Superfusion of 5 microM NE caused elevations of the spontaneous firing rate. Superfusion of 30 microM NE caused depressions, which were occasionally preceded by an excitation. Iontophoretic application of NE to grafted Purkinje neurons primarily caused depressions of the spontaneous discharge rate. Thus, the NE-induced excitations previously reported from in vitro slices are not anomalies of the in vitro slice preparation, but can be observed with superfusion of NE in our in vivo preparation as well. In general, the excitations caused by low doses of superfused NE were blocked by timolol, a specific beta-adrenergic antagonist, while the depressions caused by 30 microM superfused NE or iontophoretically applied NE were blocked by the specific alpha-adrenergic antagonist phentolamine. Large doses of sotalol were found to block both excitatory and depressant responses while lower doses only antagonized the NE-induced excitations. Taken together, these results suggest that the inhibitory effects of NE on PUrkinje neuron firing rate in intraocular cerebellar grafts in vivo are mediated via an alpha-adrenergic receptor mechanism, while the excitations caused by NE may be beta-mediated.

Age-related reduction in responses of rat hippocampal neurons to locally applied monoamines

Age-related changes in the responsiveness of hippocampal pyramidal neurons to norepinephrine (NE) and serotonin (5HT) were investigated using electrophysiological techniques. Local application of each monamine via pressure micro-ejection was employed to establish the dose which elicited a 50% change in spontaneous discharge rate of single pyramidal neurons; these data were used to construct dose response curves for the population of neurons tested in 3-6, 11-13, 18-20, and 27-30 month old rats. The percentage of cells responding in rats 18-20 and 27-30 months old decreased for both NE and 5HT. There was also a progressive increase with age in the population ED50 for 5HT starting at 18-20 months. For neurons which demonstrated a response to NE, no decrease in the population ED50 was observed. Taken together these data indicate that there is a progressive age-related decline in the postsynaptic response to NE and 5HT in the rodent hippocampus.

Selective antagonism of nicotine actions in the rat cerebellum with alpha-bungarotoxin

Nicotine, locally applied to identified neurons in the rat cerebellar cortex, excites inhibitory interneurons, but depresses the discharge of Purkinje cells. Alpha-bungarotoxin blocked the excitatory actions of nicotine on the inhibitory interneurons. The antagonism of nicotine excitatory actions is largely irreversible and also insurmountable with higher doses of nicotine. The antagonism by alpha-bungarotoxin is, in addition, selective since there is no blockade of the inhibitory actions of nicotine on Purkinje neurons. The present data suggest that the excitatory actions of nicotine on inhibitory interneurons are mediated by neuromuscular-type nicotinic receptors in the cerebellum. Moreover, the present data also supports the hypothesis of multiple nicotinic sites of action in mammalian brain.

Characterization of the techniques of pressure ejection and microiontophoresis using in vivo electrochemistry

Catecholamine levels in frontal cortex were determined in vivo by electrochemical detection after the local application of dopamine (DA) from multibarrel micropipettes by pressure ejection or microiontophoresis. Tissue DA levels were linearly related to microapplication doses with either technique and reached steady state with longer application times. Furthermore, the plateau DA tissue concentrations were clearly related to ejection pressure or iontophoretic current. Using either microapplication technique, the tissue DA levels decreased as distance between the recording electrode and the tip of the drug pipette increased. However, pressure ejected and iontophoretically applied drug differed in their concentration versus time dynamics. Thus, although similar tissue concentrations of drug can be generated by the two techniques, the time dynamics of the drug effects may not be comparable. The quantitative use of these drug application techniques requires a minimal amount of variance in release between pipettes in order to effectively measure small sensitivity differences. Although the 10-fold variance with microiontophoresis does not appear resolvable at present, improved pipette construction techniques permit the variability in dosage to be limited to a maximum of 3-fold with pressure ejection. In addition, the present data also suggest that this variance can be further minimized by holding either ejection duration or ejection pressure constant when establishing dose-response relationships.

Differential effects of norepinephrine on hippocampal complex-spike and theta-neurons

The central noradrenergic system has long been postulated to modulate learning and memory. A brain structure known to be important in these functions is the hippocampus. Since the hippocampus receives a noradrenergic projection from the locus coeruleus, knowledge of norepinephrine's actions in the hippocampus may help determine its role in learning and memory. In the present study, the effects of norepinephrine were examined on two hippocampal cell types: complex-spike and theta-neurons. In the hippocampus, there is good evidence that complex-spike cells are pyramidal neurons, while theta-neurons are interneurons. Extracellular action potentials from hippocampal neurons were recorded using multibarrel glass micropipettes. Drugs were locally applied using pressure micro-ejection. Norepinephrine inhibited the spontaneous firing of complex-spike cells, while theta-neurons were excited. The inhibitory response of complex-spike neurons was mediated by an alpha 1-receptor. However, selective agonists for the alpha 2- and beta-noradrenergic receptors excited the complex-spike cells. The noradrenergic-induced excitatory response of theta-neurons was also mediated by alpha 2- and beta-receptors. This study provides evidence that locally applied norepinephrine produces different responses on two types of hippocampal neurons. Furthermore, these differential responses arise primarily from the activation of distinct populations of noradrenergic receptors.

The electrophysiological effects of nicotine in the rat cerebellum: evidence for direct postsynaptic actions

The agonistic actions of nicotine in the cerebellum were dependent on the type of cerebellar neuron studied. Purkinje cells were inhibited and cerebellar interneurons were excited by pressure-ejected nicotine. The simultaneous iontophoresis of Mg2+ failed to block nicotine agonistic actions on either cell type. Since Mg2+ reduces presynaptic release of neurotransmitters, our findings suggest that the pharmacological actions of nicotine on cerebellar neurons are due to direct postsynaptic mechanisms.

Genetic correlation of ethanol-induced ataxia and cerebellar Purkinje neuron depression among inbred strains and selected lines of rats

In the present study, we compared phenotypic differences in behavioral and neurophysiological responses to acute ethanol administration among six inbred rat strains. Genetic variation was found both for ataxia, as measured by loss of righting response (sleep time) after a hypnotic dose of ethanol, and for the depressant action of ethanol on the spontaneous discharge of cerebellar Purkinje neurons. Results from an analysis of covariance of these phenotypes, measured among the inbred strains, provided strong evidence for a high genetic correlation between sleep time and inhibition of cerebellar Purkinje neuron discharge in response to acute ethanol administration. However, ethanol metabolism was also found to correlate with the behavioral sensitivity of rats to ethanol. Preliminary data from the third generation of replicate lines of rats currently being selectively bred for high and low acute sensitivity to ethanol shows a trend toward divergence of both ethanol sleep time and neuronal sensitivity to acute ethanol. The conclusion from these data supports the hypothesis that the cerebellum is an important locus of ethanol action, and suggests that neuronal sensitivity to ethanol will continue to diverge between these rat lines as selection for the sleep time phenotype progresses.

Calcium differentially alters behavioral and electrophysiological responses to ethanol in selectively bred mouse lines

Sensitivity to the hypnotic action of ethanol has been found to increase in SS/Ibg (SS) but not in LS/Ibg (LS) mice after intracerebroventricular (icv) administration of calcium. In the present investigation, a correlation was found between calcium-induced changes in behavioral sensitivity and in the sensitivity of cerebellar Purkinje neurons to the depressant effects of locally applied ethanol. Cerebellar Purkinje neuron sensitivity was measured as the dose of ethanol pressure ejected from a multibarreled micropipette required to produce a 50% depression of spontaneous firing rate of single neurons. Administration of 0.2-0.4 mumol calcium chloride into the lateral ventricle of the brain increased the sensitivity of SS but not LS mice to the hypnotic behavioral effect of systemically administered ethanol. Similarly, Purkinje neuron sensitivity to locally applied ethanol was also enhanced in SS but not in LS mice 15 min following administration of calcium (0.25 mumol) icv. Furthermore, locally applied ethanol was more effective in depressing spontaneous Purkinje neuron discharge in SS mice when a 1 mM calcium solution was concomitantly pressure ejected with ethanol from the micropipette. Magnesium chloride did not mimic the effects of calcium on either behavioral or electrophysiological effects of ethanol, suggesting that the action of calcium is not a nonspecific effect of divalent cations. These data suggest that calcium-dependent processes may be involved in behavioral and electrophysiological effects associated with ethanol intoxication. Further research will be required to determine if the genetically selected difference in ethanol sensitivity expressed in LS and SS mice is regulated by calcium mechanisms.

Adrenergic responses of baroreceptive cells in the nucleus tractus solitarii of the rat: a microiontophoretic study

Anatomical and pharmacological evidence suggests a role for catecholamines (CAs) in the modulation of the baroreceptor reflex within the nucleus tractus solitarii (NTS). Single neurons in the NTS of the rat were studied for their responses to activation of the baroreceptor reflex and to iontophoretic administration of dopamine, norepinephrine (NE), and epinephrine (EPI) to determine the relationship between the effects of baroreflex activation and CA application on baroreceptive neurons in the vagal sensory nucleus. Of 269 cells studied, 104 (38.7%) exhibited decreases and 41 cells (15.2%) showed increases in firing rate in response to baroreflex activation, while the remaining 124 neurons showed no response. All 3 CAs inhibited spike activity in the majority (68.5%) of NTS cells. These inhibitory effects on spontaneous firing were observed regardless of the response profile of a particular neuron to baroreflex activation. The inhibitory effects of NE and EPI on NTS neuronal activity were specifically blocked by the alpha-adrenergic receptor antagonist tolazoline, but not by the beta-adrenergic antagonist sotalol. These results indicate that CAs may interact at several sites within the NTS to influence baroreflex integration, and that the effects of NE and EPI on neuronal activity are mediated by an alpha-adrenergic receptor.

Desipramine and noradrenergic neurotransmission in aging: failure to respond in aged laboratory animals

Deficiencies in noradrenergic neurotransmission have been found in the central nervous system of aged laboratory animals. The purpose of the present study was to determine if tricyclic antidepressants, such as desipramine, can overcome the diminished noradrenergic neurotransmission found in these animals. Using electrophysiological techniques, noradrenergic neurotransmission was examined in the cerebellar cortex of rats, a model system which has been used extensively to characterize the effects of norepinephrine in the central nervous system. The discharge rate of cerebellar Purkinje neurons is very sensitive to changes in the noradrenergic input from the nucleus locus coeruleus. In this model system in young rats, treatment with desipramine slowly augments noradrenergic neurotransmission over several weeks. Similar treatment in aged animals caused no increase in the age-related deficient noradrenergic neurotransmission. The decline in efficacy of desipramine with age could not be accounted for by differences between young and old rats in the distribution of the drug. Failure of desipramine to be effective in older rats may reflect the insensitivity of aged neurons to norepinephrine itself, so that treatment strategies which increase the amount of nerepinephrine released onto these neurons may be ineffective. The findings may have implications for the use of tricyclic antidepressants in aged depressed patients.

Interaction of serotonin with somatosensory cortical neuronal responses to afferent synaptic inputs and putative neurotransmitters

The present study was conducted to investigate the action of serotonin (5-HT) on synaptic transmission within local circuits of the rat somatosensory cortex. Responses of single somatosensory cortical neurons to activation of excitatory and inhibitory synaptic pathways or iontophoretic application of putative neurotransmitters were examined before, during and after microiontophoresis of 5-HT. Monoamine-induced changes in neuronal responsiveness were quantitatively assessed by computer-based analysis of peri-event histograms. 5-HT typically exerted a differential inhibitory effect on neuronal firing, such that stimulus-induced responses were reduced relative to spontaneous discharge. In 16 of 24 (67%) of the cells tested, 5-HT depressed synaptically evoked excitation more than background firing such that "signal to noise" ratio was decreased. In some cases evoked spiking was reduced from control levels at doses of 5-HT subthreshold for producing direct depression of baseline firing rate. Cortical neuron excitatory responses to iontophoretically applied acetylcholine (8 of 13 cells) and glutamate (10 of 15 cells) were also reduced during microiontophoresis of 5-HT. A similar reduction in inhibitory efficacy was observed in 62% of the cases (10 of 16 cells) where 5-HT was interacted with GABA-induced depressant responses. Local administration of 5-HT also resulted in an antagonism of stimulus bound inhibition of firing (9 of 11 cells). These results are contrasted with previously observed facilitory effects of norepinephrine (NE) on cortical neuronal responsiveness to afferent synaptic inputs and putative transmitter agents. It is suggested that endogenously released 5-HT and NE may exert complementary modulatory-type actions on neuronal responsiveness as a means of regulating the transfer of sensory information through local cerebrocortical circuits.

Interactions of phencyclidine with hippocampal circuitry: evidence for neuronal heterogeneity

The discovery of phencyclidine (PCP) receptors has stimulated the search for specific PCP antagonists. A direct product of this research is metaphit, an irreversible PCP ligand, which has recently been synthesized. In this study we examined the effects of metaphit on the responses of hippocampal neurons to PCP. On the basis of unfiltered action potential durations, hippocampal cells were divided into two groups, complex-spike cells and theta neurons. Local application of PCP caused inhibitions of the spontaneous firing rates of complex-spike cells. Metaphit, locally applied, antagonized approximately 50% of these responses, while the remaining responses were unaffected. In contrast, PCP caused increases in the spontaneous firing rates of theta cells and in almost all cases, these responses to PCP were attenuated by metaphit administration. These effects of metaphit were specific for PCP as the responses to locally applied norepinephrine were not altered by metaphit. The data suggest two mechanisms of action of PCP in the hippocampus. In addition, these mechanisms may be localized in part to different cell types.

Antagonism of phencyclidine action by metaphit in rat cerebellar Purkinje neurons: an electrophysiological study

Metaphit (1-[1-(3-isothiocyanatophenyl)-cyclohexyl]piperidine), a derivative of the psychotomimetic drug phencyclidine (PCP), is postulated to bind irreversibly to PCP receptors. We examined here the electrophysiological interactions of metaphit with PCP in rat cerebellar cortex, since a specific effect of PCP on cerebellar neuronal circuitry has been shown. Metaphit, applied locally to Purkinje neurons by micropressure ejection through multibarreled micropipettes, has a reversible depressant action lasting for 5-20 min. Following this, PCP-induced inhibition is blocked with no recovery despite repeated applications of PCP for over an hour. This blockade was not seen unless the dose of metaphit was sufficient to transiently depress Purkinje neuron discharge. Metaphit does not antagonize inhibitory effects of locally applied norepinephrine or gamma-aminobutyric acid. This electrophysiological data suggests that metaphit is an irreversible antagonist of PCP in the cerebellum.

Enhanced sensitivity of cerebellar Purkinje cells to iontophoretically-applied serotonin in thiamine deficiency

Electrophysiological aspects of thiamine depletion in the rat induced by dietary deficiency are described. Behavioral changes as well as qualitative and quantitative alterations in the sensitivity of cerebellar Purkinje cells to iontophoretically-applied 5-hydroxytryptamine (5-HT) were observed. Thiamine-deficient rats were characterized essentially by ataxia, piloerection, paresis, apparent weakness, and hypothermia after 4-6 weeks on a thiamine-free diet. Basal Purkinje cell firing frequency was unaffected by thiamine deficiency. The response of Purkinje cells to iontophoretically-applied 5-HT was solely inhibitory in deficient rats. In control rats, however, responses to 5-HT were excitatory, biphasic, or inhibitory. Neurons in the thiamine-deficient animals were more sensitive to the inhibitory effects of 5-HT, as demonstrated by a significant parallel shift to the left of the dose-response curve. Durations of 5-HT effects were similar in both groups. Dose-response relationships for GABA-induced inhibition of Purkinje cell firing from thiamine deficient and control rats did not differ from one another. These data demonstrate a relatively selective effect of thiamine depletion on cerebellar serotonergic neurotransmission assessed electrophysiologically. We believe there is up-regulation of 5-HT receptors on Purkinje cells caused by thiamine deficiency-induced impairment of indoleamine input to the cerebellum from raphe and related nuclei.

Norepinephrine elicits both excitatory and inhibitory responses from Purkinje cells in the in vitro rat cerebellar slice

Superfusion of Purkinje neurons in the in vitro rat cerebellar slice with norepinephrine caused increases and decreases of spontaneous Purkinje cell firing. Excitations were evoked by low concentrations of norepinephrine (0.5-10 microM) and by the beta receptor agonist isoproterenol (0.1-5 microM). These excitations were reduced by timolol (1-2 microM), a beta receptor antagonist. Perfusion with higher concentrations of norepinephrine (greater than 16 microM), caused a depression of Purkinje neuron spontaneous activity. This inhibitory response was blocked by the alpha receptor antagonist phentolamine. The alpha 1 selective agonist phenylephrine had no effect on spontaneous activity at concentrations up to 100 microM, but the alpha 2 selective agonist clonidine (1-50 microM) elicited decreases in firing rate. These responses appeared to be due to a direct action on Purkinje cells, because neither the excitation nor the depression of Purkinje neuron activity elicited by norepinephrine was substantially altered when tested in a medium which substantially blocked synaptic transmission within the slice. Under these in vitro conditions, norepinephrine appears to increase the firing rate of Purkinje neurons via an interaction with beta adrenergic receptors, while norepinephrine induced depressions may be linked to alpha adrenergic receptor interactions; both receptors appear to be located directly on the Purkinje neurons.

Structural and Functional Impairment of Adrenergic Input to Intraocular Cerebellar Grafts During Thyroid Hormone Deficiency

Embryonic cerebellae were transplanted to the anterior eye chamber of normal and thyroidectomized adult rats and were left in the eye to mature for 5 weeks. The cerebellar grafts received adrenergic innervation from the sympathetic ground plexus of the host iris. The density of adrenergic fibers innervating the grafts was reduced by approximately 50% in all thyroidectomized groups, without any effect on the fluorescence intensity or the morphology of individual nerve fibers. The reduction in adrenergic ingrowth was entirely prevented in grafts raised in similarly thyroidectomized recipients, which were substituted daily with thyroxine (100 micrograms/kg, SC). Electrophysiologically, there was no difference between the groups in terms of spontaneous firing rate of Purkinje neurons, recorded extracellularly from cerebellar grafts. However, there was a 10-fold decrease in the potency of catecholamines to inhibit the spontaneous neuronal activity in thyroidectomized hosts as compared to controls when applied either locally by micropressure ejection or by superfusion. Similar to the observed histologic changes in catecholamine innervation, the abnormal responsiveness of hypothyroid cerebellar neurons to locally applied catecholamines could be prevented by daily substitution with thyroxine in these animals. It is thus concluded that there is both structural and functional impairment of the adrenergic innervation of intraocular cerebellar grafts that underwent development under conditions of thyroid hormone deficiency.

Noradrenergic action in the developing rat cerebellum: interaction between norepinephrine and gamma-aminobutyric acid applied microiontophoretically to immature Purkinje cells

In this study, we examined the action of norepinephrine (NE) on developing cerebellar Purkinje cells. Responses of immature Purkinje cells to microiontophoretically applied NE were assessed both in terms of the direct depressant effect of the catecholamine as well as its ability to interact synergistically with gamma-aminobutyric acid (GABA). Sensitivity for NE, GABA, taurine and beta-alanine was found to be present by postnatal day 3. In addition, NE enhanced inhibitory responses of Purkinje cells to GABA but not those to beta-alanine by at least postnatal day 5, before periods of extensive morphological differentiation and synaptic investment. The modulation by NE persisted into adulthood with no qualitative changes in characteristics. The results of this study support and extend the hypothesis that 'chemosensitivity' antedates synaptogenesis' to include responsiveness of developing Purkinje cells to interactions between putative cerebellar neurotransmitters.

Differential sensitivity of cerebellar purkinje neurons to ethanol in selectively outbred lines of mice: maintenance in vitro independent of synaptic transmission

The effects of ethanol on spontaneous firing of cerebellar Purkinje neurons were examined in outbred lines of mice (short-sleep, SS; and long-sleep, LS) which exhibit differential behavioral sensitivity to ethanol. In order to determine whether the differences in Purkinje cell ethanol sensitivity which are observed in situ reflect differences in intrinsic properties of Purkinje neurons, we developed an isolated in vitro preparation of mouse cerebellum. Even when synaptic transmission was largely inhibited by elevating Mg2+ and decreasing Ca2+ concentrations, Purkinje cells demonstrated stable long-term firing rates quite similar to those observed in vivo. Purkinje cells responded to superfusion of ethanol with both increases and decreases in firing rate. Inhibition of rate was more commonly observed, and was the only response which was demonstrably dose-dependent. The differential sensitivity to ethanol which we have previously reported in vivo was maintained even under under these conditions, with the LS mice being approximately 5 times more sensitive to the depressant effects of ethanol. In addition, it was shown that ethanol, at the concentrations used in these experiments, decreased the amplitude and increased the duration of single action potentials. Thus, taken together, these results suggest that the differential sensitivity of outbred lines to the soporific effects of ethanol are paralleled by differences in the sensitivity of Purkinje neurons in vitro to superfusion with ethanol. Because these differences can be observed even when synaptic transmission is largely suppressed, it would appear that these differences are intrinsic to the purkinje neurons themselves.

Cultured cerebellar neurons: endogenous and exogenous components of Purkinje cell activity and membrane response to putative transmitters

Modified explant cultures of fetal rat cerebellum were developed for electrophysiological and pharmacological studies, at the membrane level, of Purkinje neurons. The goals of the present series of experiments were to identify possible endogenous and exogenous components to the electrical activity of Purkinje neurons, to assess the sensitivity of these neurons to putative excitatory and inhibitory neurotransmitters, and to characterize the membrane response to the transmitters. Intracellular recordings were made from Purkinje neurons, identified on a morphological basis, using conventional electrophysiological techniques. Virtually all Purkinje neurons displayed spontaneous activity. A contribution of both endogenous and exogenous components to the spontaneous activity was indicated by alterations in the pattern and amount of activity when the membrane potential was varied and by the characteristics of the individual potentials themselves. Several types of activity were considered to be endogenous: the most common type consisted of pacemaker-like potentials which generated a pattern of firing similar to that characterized as simple spike activity in previous in vivo studies; another type of endogenous activity consisted of large membrane depolarizations that evoked one or two spikes. These depolarizing responses were similar to the membrane response generated by climbing fiber input to Purkinje cells in vivo. The exogenous components to the spontaneous activity consisted of synaptic potentials including excitatory (EPSPs) and inhibitory (IPSPs) synaptic potentials and biphasic EPSP/IPSPs. Several putative transmitters thought to mediate these synaptic potentials were tested by focal micropressure application to determine if they could mimic the action of the endogenous transmitters. The putative transmitter glutamate depolarized the cultured Purkinje neurons and evoked action potentials, characteristics which were displayed by the excitatory synaptic potentials. The putative inhibitory transmitter GABA hyperpolarized the cultured Purkinje neurons and depressed activity, characteristics which were displayed by the inhibitory synaptic potentials. The putative inhibitory transmitters glycine and taurine were ineffective. Norepinephrine, the transmitter mediating the inhibitory input from the locus coeruleus to Purkinje neurons, was also tested. When applied in the microM range, NE effects were variable. When applied in the mM range, NE depressed the spontaneous activity in a manner suggestive of a presynaptic action.

Trophic effects of brain areas on the developing cerebral cortex: II. Electrophysiology of intraocular grafts

Electrophysiological correlates of locus coeruleus-induced growth stimulation in rat cortical grafts homologously transplanted to the anterior chamber of the eye were studied. Neurons in growth-stimulated grafts manifested a slow sustained spontaneous discharge similar to that found in rat cortex in situ. Local administration of glutamate markedly augmented this discharge. In contrast, neurons from nonstimulated grafts fired in high frequency bursts separated by long pauses, and this discharge was comparatively insensitive to glutamate. Poststimulus inhibition after local stimulation of the transplant surface was readily observed in the growth-stimulated grafts, but absent in all non-stimulated grafts tested. Moreover, superfusion of picrotoxin, which antagonizes GABA-mediated inhibitory pathways, reversibly converted the growth-stimulated graft discharge pattern into one characteristic of non-stimulated grafts. Taken together with the data in the preceding paper, the results demonstrate the importance of extrinsic inputs for functional development of neuronal circuits within neocortex.

The effect of local application of homocysteine on neuronal activity in the central nervous system of the rat

Homocysteine, a monocarboxylic, sulfur-containing amino acid, produces convulsions in rats and mice when administered systemically. Convulsions and high serum concentrations of homocysteine are among the symptoms that characterize patients with homocystinuria, a hereditary disorder of amino acid metabolism. In order to evaluate the effects of homocysteine on the central nervous system directly, extracellular recordings were made from neurons in rat cerebral cortex, cerebellum and midbrain during local application of homocysteine by pressure ejection or iontophoresis. Both methods of drug delivery produced dose-dependent increases in the activity of neurons in every area tested. Activity was increased by D,L-homocysteine and L-glutamate in 67 percent of cells tested with both drugs. The doses required to produce equivalent excitations in this group of cells were similar, suggesting that homocysteine is at least as potent as glutamate. The excitatory effects of both homocysteine and glutamate were antagonized by local application of betaine, a biological methyl donor which blocks convulsions produced by systemic administration of pentylenetetrazol and electroshock as well as homocysteine. The effects of local application of homocysteine were also blocked by local application of the glutamate antagonist glutamate diethylester (GDEE). In 6 of 7 cells tested, GDEE appeared to preferentially affect homocysteine-induced excitations. These data indicate that homocysteine has an excitatory action on neurons, a finding which may account for some of the symptoms associated with certain disorders of amino acid metabolism.

Ethanol and some tetrahydroisoquinolines alter the discharge of cortical and hippocampal neurons: relationship to endogenous opioids

The activity of single neurons in rat cortex or hippocampus (HPC) was recorded to test two hypotheses: (1) neuronal effects of ethanol are mediated by an endogenous opiate-like mechanism (for example, by release of an endogenous opioid peptide), and; (2) ethanol-induced formation of aldehyde-catecholamine condensation products (tetrahydroisoquinolines; TIQs) might contribute to some acute actions of ethanol. Ethanol and all TIQs were applied to single neurons from multibarrel micropipettes by electroosmosis or pressure. Ethanol most often inhibited neurons of the parietal cortex, while activating most HPC pyramidal neurons. Tetrahydropapaveroline (THP) most often inhibited the spontaneous and glutamate- or acetylcholine (ACh)-induced firing of neurons in both these regions, although some excitations were also seen. In contrast, salsolinol and 7-O-methyl-salsolinol predominantly excited HPC pyramidal neurons, but depressed most parietal cortical neurons. Iontophoretic or SC naloxone usually antagonized the excitatory actions of ethanol, salsolinol and methionine5-enkephalin on HPC pyramidal cells; however, ACh-induced speeding also was antagonized occasionally. Conversely, the antimuscarinic agent scopolamine antagonized the excitatory actions of salsolinol, but not those of met-enkephalin, in some HPC pyramidal cells. These results and those of previous studies show that acutely applied ethanol or salsolinol elicits a region-specific pattern of neuronal effects in brain similar to that previously described for opiates: activity is inhibited in several tested brain areas but excited in hippocampus. Furthermore, these excitatory effects are antagonized by naloxone. However, because of the occasional apparent non-specific effects of naloxone and the puzzling antagonism of the salsolinol-induced excitations by scopolamine, some doubt remains whether the opiate-like actions of these substances can be completely attributed to mediation by opiate receptors.

Evidence for genetic correlation of hypnotic effects and cerebellar Purkinje neuron depression in response to ethanol in mice

In the present study, we compared phenotypic differences in behavioral and neurophysiological responses to acute ethanol administration among eight inbred strains of mice. Genetic variation for behavioral sedation, as measured by loss of the righting reflex (sleep time) after a hypnotic dose of ethanol, was shown to be present among the inbred strain population. In addition, a large genetic component of variation in the depressant action of ethanol on the spontaneous discharge of cerebellar Purkinje neurons was found. Results from an analysis of covariance of the behavioral and electrophysiological phenotypes, measured on each mouse among the inbred strains, provided strong evidence for a high genetic correlation between sleep time and inhibition of cerebellar Purkinje neuron discharge in response to acute ethanol administration. Taken together with our previously reported data on ethanol-induced electrophysiological changes in selectively bred lines, the results described here strongly support the hypothesis that the cerebellar Purkinje neuron is one important locus for the acute soporific effects of alcohol.

Stereoselectivity of opiate antagonists in rat hippocampus and neocortex: responses to (+) and (-) isomers of naloxone

The relative potencies of the (+) and (-) isomers of naloxone in antagonizing electrophysiological responses to D-alanine2-methionine enkephalinamide were compared in rat frontal cortex and hippocampus. In the in vitro hippocampus, the (-) isomer was found to be at least a 100 times more potent than the (+) isomer in antagonizing opiate-induced changes in field potentials. Similar stereoselectivity was observed in vivo in both frontal cortex and hippocampus in terms of the antagonism of enkephalin-induced changes in spontaneous cell firing. The direct effects of (+) and (-)-naloxone were examined as well. In hippocampus both in vivo and in vitro, no differential effect was observed, whereas in the neocortex (-)-naloxone was considerably more potent than the (+) isomer in eliciting depressions of spontaneous activity. These direct effects of naloxone in the cortex do not appear to be due to an antagonism of the effects of endogenously released opioids. These results demonstrate that the stereoselectivity of naloxone isomers in antagonizing electrophysiological responses to opiates in the cortex and hippocampus parallels that previously observed in other brain regions and in other tissues. In addition, they suggest that naloxone may have interactions with other unknown opiate (or possibly non-opiate) receptors which are of physiological significance.

Hippocampal noradrenergic responses in vivo and in vitro. Characterization of alpha and beta components

Pressure ejection of l-norepinephrine (NE) in the in vivo rat hippocampus generally produced depression of pyramidal cell spontaneous activity. In addition, both excitation and biphasic responses were observed. NE-induced inhibition of firing rate was effectively antagonized by concurrent administration of the alpha antagonist phentolamine, but was largely unaltered by the beta antagonist timolol. On the other hand, NE-induced elevation in spontaneous firing rate was effectively blocked by timolol, and largely unaffected by phentolamine. Another beta antagonist, sotalol, did not selectively antagonize either NE-induced inhibition or NE-induced excitation. The beta agonist 2-fluoro-NE produced increases in pyramidal cell firing rates in most cells studied, while the alpha agonist 6-fluoro-NE inhibited the majority of cells examined. The effects of sotalol were also examined on alpha and beta receptor-mediated field responses in the in vitro hippocampal slice. Sotalol was shown to be a selective beta antagonist in this system, blocking excitation evoked by the beta agonist isoproterenol while having no effect on inhibition elicited by the alpha agonist clonidine; however, the potency of sotalol (Ki = 3.5 microM) was considerably less than that of timolol (Ki = 50 nM). Taken together, these results suggest that NE-induced depression and elevation in hippocampal pyramidal cell spontaneous discharge in vivo are mediated via alpha and beta adrenoceptors, respectively.

Interactions of a neuroleptic drug (fluphenazine) with catecholamines in hippocampus

The interactions of fluphenazine with the electrophysiological responses to catecholamines were studied in the rat hippocampus and parietal cortex. In the in vitro hippocampal slice, changes in synaptically evoked responses induced by norepinephrine, isoproterenol and dopamine were not altered by superfusion of fluphenazine. Both alpha- and beta- components of adrenergic responses were unaffected by neuroleptic administration in this preparation. Similarly, alterations in the spontaneous firing of single hippocampal pyramidal neurons in situ to adrenergic agonists or dopamine were not affected by local fluphenazine and administration using pressure ejection through multibarreled micropipettes. In contrast, norepinephrine- or isoproterenol-induced inhibitions of parietal cortical neurons in situ were potently antagonized by fluphenazine. A similar interaction was observed from a hippocampal basket neuron. It is concluded that while fluphenazine can antagonize well-defined noradrenergic effects in some brain regions (e. g., cerebellum, cortex), this property is not generalized to all brain regions receiving noradrenergic input.

Benzodiazepines and central inhibitory mechanisms

The effect of diazepam was evaluated on spontaneous activity and drug- and electrically-elicited inhibitions of neuronal activity. Doses of diazepam which did not change spontaneous firing rates markedly enhanced GABA-mediated inhibitions in rat cerebellum in situ and in tissue cultures of rat hypothalamus. The effects of diazepam were readily reversible, and could be antagonized by picrotoxin; no effect on glycine or norepinephrine-induced inhibition was seen. It is concluded that actions of diazepam are mediated, at least in part, by a specific increase in GABA-mediated inhibition in the central nervous system.

Effect of dopamine agonists and antagonists on the electrical activity of substantia nigra neurons transplanted into the lateral ventricle of the rat

Pieces of fetal rat mesencephalon containing the substantia nigra were transplanted into the lateral ventricle of rats pretreated with 6-OHDA injections into the ipsilateral substantia nigra. Extracellular recordings from these grafts revealed spontaneously active neurons with action potential waveforms and firing rates similar to those found in substantia nigra zona compacta in situ. Furthermore, local application of dopamine agonists inhibited, while dopamine antagonists increased zona compacta activity, both in situ and of cells from nigral grafts. We conclude that substantia nigra grafts contain spontaneously active neurons similar to those of zona compacta in situ, and that some of the mechanisms regulating nigral neuronal activity in situ are also present in this preparation.

Differential electrophysiological and behavioral responses to optically active derivatives of phencyclidine

Dextro- and levorotatory isomers of 1-(1-phenylcyclohexyl)-3-methylpiperidine (PCMP) were synthesized. Both isomers inhibited spontaneous cerebellar Purkinje neuron firing when applied locally by pressure ejection. This effect was dose-dependent, with the (+)-isomer about 5--7 times more potent than the (-)-isomer. Both isomers also depressed rotarod performance in mice. Again, the (+)-isomer was about 5 times more potent than the (-)-isomer. Both rotarod performance and Purkinje cells discharge were depressed maximally 10--15 min after i.p. injection of drug. Our results suggest a correlation between behavioral performance and central neuron electrophysiological activity and suggest that the central actions of PCP or its derivatives are probably mediated at one locus, by a stereospecific mechanism.