Inhibiton of neurons in the amygdala by dorsal raphe stimulation: mediation through a direct serotonergic pathway

Electrophysiological analysis of inhibitory synaptic mechanisms in the preoptic area of the rat

1. Extracellular recordings were made from single neurones in the medial preoptic-anterior hypothalamus of rats anaesthetized with urethane or pentobarbitone 2. Stimulation of the arcuate--ventromedial area evoked inhibition in 76% of neurones; inhibition occurred as the initial response in 26% of neurones, followed orthodromic excitation in 26% and antidromic excitation in 24%. Stimulation of the periaqueductal grey evoked inhibition as the initial response in 63% of neurones. 3. Stimulation of both the arcuate--ventromedial area and the periaqueductal grey generated synaptically evoked high-frequency discharges in a few neurones. 4. Short ionophoretic pulses of beta-alanine, gamma-aminobutyric acid (GABA), glycine and taurine inhibited the discharge of most preoptic-anterior hypothalamic neurones. Bicuculline methochloride and picrotoxin antagonized both GABA responses and synaptic inhibition. Strychnine antagonized beta-alanine, glycine and taurine responses without altering synaptic inhibition. 5. Reduction of hypothalamic serotonin (5-HT) levels to about 40% of control values by micro-injection of 5,7-dihydroxytryptamine into the ventral tegmental area, or parenteral injection of parachlorophenylalanine, did not alter the profile of responses evoked by either arcuate-ventromedial or periaqueductal grey stimulation. Micro-injection of tetanus toxin (100-200 MLD) into the preoptic-anterior hypothalamus significantly reduced the frequency of occurrence of inhibition evoked from both sites. 6. Pentobarbitone administered acutely or as the sole anaesthetic increased both the duration and frequency of occurrence of inhibitory responses evoked from the arcuate-ventromedial nuclei. 7. It is suggested that a GABA-linked inhibitory synaptic mechanism, activated by arcuate-ventromedial and periaqueductal grey stimulation, operates in the preoptic-anterior hypothalamus. Such convergent inhibition may be a result of activity in local interneurone circuitry.

Reciprocal connections between the lateral hypothalamus and the frontal complex in the rat: electrophysiological and anatomical observations

Inputs to rat lateral hypothalamus (LHA) from prefrontal cortex (FC), and vice versa, were studied by intracellular recording, and by retrograde horseradish peroxidase (HRP) method. Stimulation of the FC evoked 3 types of responses: a polysynaptic EPSP-IPSP sequence, IPSPs alone, or antidromic response in LHA neurons. Forty-five per cent of IPSPs were considered to be monosynaptic since the latencies were constant when stimulus intensity was changed. The neurons labeled in the FC following electrophoretic injections of HRP into LHA were located in the medial and sulcal FC. In these cortical areas, not only pyramidal neurons in layer V, but also non-pyramidal neurons in layer VI were labeled. Stimulation of the LHA evoked an EPSP-IPSP sequence, or antidromic response in FC neurons. Some of the fast EPSPs were considered to be monosynaptic. The neurons labeled in the LHA following HRP injection into the FC were either relatively large spherical neurons or small ovoid-shaped neurons. These were distributed diffusely throughout the LHA.

Effects of stimulation of the raphe nuclei on muricide behavior in rats

Mouse-killing behavior was induced in male Wistar rats by chronic isolation. Selective stimulation of the dorsal raphe nucleus markedly reduced this aggressive behavior. On the other hand, stimulation of the median raphe nucleus produced no changes in muricide behavior. Our results indicate that dorsal and median raphe nuclei function differentially in the regulation of muricide behavior with the dorsal raphe playing a critical role in this behavioral pattern.

Pharmacological characterization of serotonin receptors in the facial motor nucleus: a microiontophoretic study

The effects of 'peripheral' serotonin (5-HT) antagonists on the facilitation of facial motoneuron excitation by 5-HT were investigated in the present study. Microiontophoretic application (5--10 aA) or intravenous administration of methysergide (0.5--1.0 mg/kg), cyproheptadine (0.5--1.0 mg/kg), cinanserin (0.5--1.0 mg/kg), or metergoline (50--100 micrograms/kg) antagonized the facilitating effect of 5-HT but not that of norepinephrine (NE) on facial motoneurons. Blockade developed within 1--2 min of the intravenous administration of methysergide, cyproheptadine and cinanserin and continued up to 30 min. The antagonism produced by intravenous metergoline took 8--15 min to develop and lasted for at least 6 h. Chronic administration of metergoline resulted in supersensitivity to 5-HT and NE in the facial nucleus. The putative 5-HT antagonists chlorpromazine, propranolol, and methiothepin failed to block the facilitating effect of 5-HT; however, methiothepin did block NE. The results are discussed in relation to a two-component model of the 5-HT mediated motor syndrome consisting of : (1) patterned excitatory inputs to motor nuclei; (2) 5-HT facilitation of these excitatory inputs. The 'peripheral' 5-HT antagonists appear to directly block the latter component.

Neurochemical modulation of sensory-motor reactivity: acoustic and tactile startle reflexes

The present review argues that the startle reflex is particularly well suited as a model system to analyze how drugs alter stimulus reactivity and reflex excitability. It then reviews all the literature to date on how drugs or lesions that are thought to alter neurochemical transmitter systems affect acoustic and/or tactile startle. Hypotheses are presented to account for how serotonin, dopamine, norepinephrine, acetylcholine, and opiates modulate startle. Effects on startle plasticity such as habituation, sensitization, and potentiation resulting from prior associative learning are also included.

The action of serotonin in the rat hippocampal slice preparation

1. Intracellular activity was recorded from neurones in the CA1 pyramidal layer of slices of rat hippocampus maintained in vitro. 2. Application of 5-HT in a droplet or via ionophoresis produced a 3-5 mV hyperpolarization associated with a 30% decrease in input resistance. 3. The response to 5-HT was minimal with a drop concentration of 1 microM and maximal with 100 microM. The responses appeared to be blocked by methysergide applied in the superfusion medium. 4. The responses to 5-HT were minimal when the drug was applied in the apical dendritic region and maximal when it was applied near the soma. 5. 5-HT produced no substantial changes in e.p.s.p.s evoked by stimulation of the Schaffer collateral-commissural system or in i.p.s.p.s which were occasionally encountered following stimuli to the stratum radiatum. 6. The responses to 5-HT are true post-synaptic responses and are not indirect effects since they are present in a Ca2+-deficient Mg2+-enriched medium which blocks synaptic transmission. 7. The responses to 5-HT were not dependent on extracellular Cl- concentration. 8. These experiments indicate that 5-HT produces its effects in the rat hippocampus by activating K+ channels.

The coeruleospinal noradrenergic neurons: anatomical and electrophysiological studies in the rat

The neuranatomical location and pharmacological sensitivity of coeruleospinal neurons were studied with a combination of retrograde tracing experiments and single unit recording. Coeruleospinal neurons were multipolar, medium-sized cells and were found in the ventral division of the locus coeruleus and in the locus subcoeruleus. In the locus coeruleus proper, they presumably corresponded to the large cells of the ventral division defined in previous Golgi studies. Coeruleospinal cells were identified by antidromic stimulation from the cervical spinal cord. Their firing rate was slow and regular, their conduction velocity characteristic of unmyelinated fibers (0.65 m/sec). The method of antidromic stimulation also revealed that coeruleospinal neurones possess an anteriorly directed collateral traveling in the midbrain reticular formation outside the main noradrenergic dorsal bundle. These neurones were strongly inhibited by the iontophoretic application of morphine, noradrenaline, clonidine, GABA and excited by ACh. Although the coeruleo- and subcoeruleospinal neurones are clearly a group of cells distinct from the coeruleocortical projection, their electrophysiological and pharmacological properties are essentially identical.

Activation of lateral geniculate neurons by norepinephrine: mediation by an alpha-adrenergic receptor

Adrenergic receptors in the vicinity of neurons in the lateral geniculate nucleus (LGN) of the rat were pharmacologically characterized using extracellular single-cell recording and microiontophoretic techniques. Application of norepinephrine (NE) at low iontophoretic currents (1-15 nA) produced a delayed activation of most LGN neurons. This activation was mimicked by various sympathomimetic amines. The relative potency series of agonists was typical of postsynaptic alpha-adrenergic receptors: epinephrine greater than NE greater than phenylephrine greater than or equal to alpha-methylnorepinephrine greater than dopamine greater than isoproterenol. The alpha-antagonists phentolamine, piperoxane and WB-4101, when applied at low iontophoretic currents (less than 10 nA), produced a selective, dose-dependent and reversible blockade of the response to NE. The beta-antagonist sotalol had weak and variable effects at these currents. At low currents, the presynaptic alpha-agonist clonidine was also able to block the response to NE but, at higher currents, produced a partial activation of some units, suggesting that it is a weak agonist. The ability of sympathomimetic amines to activate LGN neurons correlates well with their reported affinities for brain alpha1-adrenoceptors labeled with [3H]WB-4101. It is concluded that NE activates neurons in the LGN via a postsynaptic or alpha1-adrenergic receptor.

Denervation supersensitivity to serotonin in rat forebrain: single cell studies

To investigate the development of denervation supersensitivity to serotonin (5-hydroxytryptamine, 5-HT) in the amygdala (AMYG) and the ventral lateral geniculate nucleus (vLGN), single cell recordings, microiontophoretic, histochemical and biochemical techniques were used in the present study. 5-HT projections to the vLGN and the AMYG were destroyed by 5,7-dihydroxytryptamine (5,7-DHT, a relatively selective toxin for 5-HT neurons) injected directly into the lateral ventricle or the ascending 5-HT pathway in the ventromedial tegmentum area. Enhanced responsiveness of cells to the inhibitory effect of microiontophoretically applied 5-HT (ionto-5-HT) began to develop within 24 h and approached a maximum 7 days after 5,7-DHT pretreatment. In general, the time courses for the reduction in both the density of 5-HT fluorescent varicosities and synaptosomal 5-HT uptake activity paralleled the time course for the development of denervation supersensitivity to 5-HT. During the first 2 days after 5,7-DHT, the enhanced sensitivity was selective for 5-HT; responses to D-lysergic acid diethylamide (LSD), norepinephrine (NE) and gamma-aminobutyric acid (GABA) were unchanged. Seven or more days after 5,7-DHT there was a marked increase of the responsiveness of neurons in the vLGN and the AMYG to both 5-HT and LSD (a 5-HT agonist which is not a substrate for the high affinity 5-HT uptake system). At these later times, the responsiveness of cells in the AMYG to NE and to a lesser extent GABA was also increased. In contrast to the marked supersensitivity seen after 5,7-DHT induced denervation, chronic administration of parachlorophenylalanine, a 5-HT synthesis inhibitor, failed to induce 5-HT supersensitivity.

Serotonergic facilitation of facial motoneuron excitation

The effect of serotonin (5-HT) on motoneurons located in the facial nucleus of the rat was investigated in the present study. Microiontophoretic application of 10--200 nA pulses of 5-HT lasting from 1 to 10 min failed to excite facial motoneurons. However, small amounts of 5-HT facilitated the subthreshold and threshold excitatory effects of iontophoretically applied glutamate on these cells. Typically, the current of glutamate required to produce an activation of facial motoneurons was reduced by at least 50% in the presence of 5-HT. In addition, 5-HT markedly shifted to the left the cumulative dose-response curve of glutamate-induced excitation of motoneurons. The 5-HT releasing agent p-chloroamphetamine (PCA) facilitated the excitatory effects of glutamate on montoneurons in control animals, but not in those pretreated with the 5-HT also facilitated the subthreshold and threshold excitation of motoneurons produced by stimulation of the motor cortex and the red nucleus. The facilitating effect of 5-HT was blocked by methysergide. Norepinephrine also facilitated facial motoneuron excitation but this effect was not blocked by methysergide. It is concluded that 5-HT in the facial nucleus functions in a manner that is not analagous to direct excitation, but rather acts as a gain setter to enhance the effects of excitatory afferent inputs.

Effects of apomorphine and sodium Di-n-propylacetate on the aggressive behaviour of three strains of mice

1. The effects of apomorphine and sodium Di-n-propylacetate (DPA, sodium valproate) on pain-induced aggressive behavior were investigated in three inbred strains of mice: BALB/c, C57B1/6 and DBA/2, which exhibited spontaneously low levels of aggression. 2. Apomorphine elicited aggressive behavior in the three strains, the range of effective doses being different for each strain of mice. 3. Di-n-propylacetate was effective in inhibiting apomorphine elicited aggression but the three strains exhibited a different sensitivity to this drug. 4. The effects of Di-n-propylacetate were not related to pain sensitivity, posture and locomotion. Only C57 strain exhibited a slight postural and locomotor impairment when injected with a higher dose of Di-n-propylacetate. 5. The results are discussed in terms of a genetic inference and of biological differences existing between these three strains.

Morphine: effects on serotonergic neurons and neurons in areas with a serotonergic input

The hypothesis that morphine acts on the serotonergic system to produce analgesia is based on the previous observations that (1) lesions and stimulation of midbrain raphe nuclei after the threshold to nociceptive stimuli; (2) morphine alters the turnover of serotonin (5-hydroxytryptamine; 5-HT). Microiontophoretic experiments were carried out to determine if morphine affected the firing rate of cells in five areas of the serotonergic system consisting of 5-HT containing neurons in the midbrain raphe nuclei (dorsal raphe and median raphe) or neurons in three areas (amygdala, optic tectum and subiculum) which are thought to receive a 5-HT input from the raphe nuclei. Morphine administered microiontophoretically slowed or inhibited the spontaneous neuronal firing in 34% of the cells studied in both the pre- and post-synaptic areas; systemically administered morphine gave similar results. However, the inhibition of neuronal firing by morphine in the five areas of the serotonergic system studied was not a specific narcotic effect because (1) it was not stereospecific; and (2) it was rarely blocked by naloxone. The inhibitory effect of morphine does not appear to be related to the analgesic effects of morphine because neuronal firing in the midbrain raphe nuclei and three possible postsynaptic areas was rarely altered by a nociceptive stimulus. It is concluded that (1) the analgesic effect of morphine is not related to an effect on neurons in the midbrain raphe nuclei and three areas which receive an identified 5-HT input; and (2) the effect of morphine on neurons in these five areas of the serotonergic system is not a specific narcotic effect.

Neuropharmacological studies on the nigro-striatal and raphe-striatal system in the rat

The responses of single neostriatal neurones to substantia nigra (SN) and dorsal raphe nucleus (DRN) stimulation and iontophoretic administration of several drugs were studied in urethane-anaesthetised rats. Stimulation of the SN-evoked excitation followed by inhibition in striatal neurones. In some cells only inhibition of firing was evoked indicating that there may be separate nigrostriatal inhibitory and excitatory pathways. DRN stimulation evoked mainly inhibition of striatal cell firing. The activity of most neurones responding to SN and DRN stimulation was depressed by iontophoretically administered dopamine, 5-hydroxytryptamine and GABA and increased by acetylcholine. Studies with antagonist revealed that alpha-flupenthixol reduced responses to dopamine and 5-hydroxytryptamine and inhibition evoked by SN and DRN stimulation. Bicuculline methochloride only reduced responses to GABA. Methysergide selectively reduced responses to 5-hydroxytryptamine and also reduced DRN-but not SN-evoked inhibition. It was concluded that the SN-evoked inhibition was probably mediated by dopamine and DRN-evoked inhibition by 5-hydroxytryptamine.

The dorsal and medial raphe projections to the substantia nigra in the rat: electrophysiological, biochemical and behavioural observations

Electrophysiological experiments have been performed in urethane anaesthetized rats to investigate the projections from the dorsal (DRN) and medial raphe nuclei (MRN) to the substantia nigra. The biochemical and behavioural effects following discrete electrolytic lesions in the dorsal and medial raphe have also been investigated. Stimulation of the DRN produced predominantly inhibition of spontaneous activity of single neurones in the substantia nigra though some neurones were also excited. Bilateral stimulation of the substantia nigra produced antidromic spikes in DRN and MRN neurones. Lesions of the DRN and MRN produced a significant reduction in substantia nigra 5-HT concentration. Additionally, DRN lesions reduced striatal 5-HT, while MRN lesions reduced hippocampal 5-HT. Both lesions increased substantia nigra HVA concentration but did not affect DA concentration. Neither DRN nor MRN lesions affected striatal HVA, although DA levels were significantly elevated after 14 days. Animals with DRN lesions explored more than controls or MRN-lesioned animals. However, this behaviour was transient and was not observed after 14 days. On the other hand, MRN-lesioned animals were significantly hyperactive. These observations suggest that the substantia nigra receives a direct monosynaptic inhibitory input from the DRN and MRN and that these pathways use 5-HT as a neurotransmitter serving to tonically inhibit dopaminergic neurones. While 5-HT and dopamine appear to be involved in the control of motor behaviour, the precise relationship between these serotoninergic and dopaminergic systems in this respect is unclear.

Antidromic identification of dopaminergic and other output neurons of the rat substantia nigra

In the present study dopamine (DA)-containing and other output neurons of the substantia nigra (SN) wer identified by antidromic stimulation from postulated target nuclei, the caudate-putamen, the thalamus, the cortex and the pontine reticular formation. To guide electrode placements, the topography of the nigrostriatal projection system was determined by retrograde tracing methods. Spontaneously active cells present in the SN were then classified in two groups according to the shape of their action potentials and their firing rate. Type I cells were located mainly in the pars compacta and could be antidromically-activated (AD-activated) from various locations along the course of the nigrostriatal pathway (caudate-putamen, globus pallidus, MFB) but not from other brain areas (ventromedial thalamus, motor cortex, pontine reticular formation). These neurons had a slow bursting pattern of firing, a very slow conduction velocity (0.58 m/sec), and a wide action potential. Their firing rate was dramatically reduced following the intravenous administration of apomorphine (ID 50: 9.3 microgram/kg), or the iontophoretic application of DA and GABA. Type II cells were located predominantly in the pars reticulata; most of them could be AD-activated from the ventromedial thalamus and the MFB but not from the motor cortex. A few of these cells could be AD-activated from the pontine reticular formation and the thalamus. A minority of type II cells, located in or near the pars compacta could be AD-activated from the caudate-putamen. In addition, their conduction velocuty was much higher (2.8 m/sec) and their firing rate far in excess of that exhibited by type I neurons. These neurons were inhibited by the iontophoretic application of GABA but not of DA. The microinjection of 6-hydroxydopamine (a neurotoxin relatively specific against catecholamine-containing neurons) in the vicinity of the MFB blocked selectively the propagation of antidromic spikes in type I but not type II cells. It is concluded that type I cells are the DA neurons of the nigrostriatal pathway. Type II cells are mainly oupput neurons that project to the ventromedial thalamus, the pons and the forebrain. This telencephalic projection most likely constitutes a second, non-DA, fast-conducting nigrostriatal pathway.

Effects of serotonin neurotoxins on rotational behavior in the rat

Injections of p-CA or 5,7-DHT into the MRN caused decreases in 5-HT and 5-HIAA levels in the cortex and striatum. They also caused a decrease in 5-HT turnover in the cortex and an increase in striatal DA turnover. The unilaterally injected rats showed contralateral rotation after amphetamine or apomorphine administration. There was a significant correlation among the rate of rotation, the decrease in cortical 5-HT turnover, and the increase in striatal DA turnover. Injection of 5,7-DHT into the SN produced the same biochemical and behavioral changes as did MRN lesions, suggesting that the changes induced by MRN lesions might be due to a direct projection from the raphe to the substantia nigra. Injection of 5,7-DHT into the MFB caused ipsilateral rotation after injection of amphetamine or apomorphine and a decrease in DA turnover in the striatum. There was a significant correlation among the rate of rotation, the decrease in 5-HT turnover in the cortex, and the decrease in striatal DA turnover in the MFB-lesioned rats. These effects might be due to a projection from the DRN to cell bodies in the striatum.

Physiological evidence for habenula as major link between forebrain and midbrain raphe

The lateral habenula is one of the few forebrain areas that project to the midbrain raphe nuclei. Electrical stimulation of the habenula markedly suppressed serotonergic neurons in the midbrain raphe. The suppression was blocked by systemic or microiontophoretic administration of picrotoxin, which suggests that gamma-aminobutyric acid is the inhibitory transmitter in the habenula-raphe pathway. These results support the concept that the habenula may serve a pivotal role in funneling information from the forebrain to the midbrain raphe.