Non-random distribution of cell types in the preoptic and anterior hypothalamic areas

Role of medial preoptic GABA neurones in regulating luteinising hormone secretion in the ovariectomised rat

The role of GABA neurones in the medial preoptic area (MPOA) in regulating the activity of the luteinising hormone-releasing hormone (LHRH) neurones projecting to the median eminence was investigated in the conscious ovariectomised rat. Plasma luteinising hormone (LH) concentrations were measured while (1) endogenous GABA release from the MPOA was monitored with the technique of microdialysis, or (2) activity at the GABA receptor was modulated by local infusions into the MPOA. Microdialysis studies revealed a fluctuating level of GABA release in the MPOA which did not correlate with pulsatile LH secretion. Infusion of 10 microM GABA (n = 8) or bicuculline methiodide (BMI, n = 6) into the MPOA, at a rate of 1 microliter/30 min, significantly inhibited mean LH concentrations (P less than 0.05-0.001) and LH pulse frequency (P less than 0.05-0.001) compared with controls (n = 8). LH pulse amplitude was not significantly altered by infusion of GABA (P greater than 0.05) while too few pulses were found after BMI treatment to enable statistical analysis. Infusions of GABA into the ventral half of the MPOA had a more significant inhibitory effect upon LH secretion compared with dorsal infusions (P = 0.012). A similar relationship did not exist for BMI infusions. These results show that acute changes in preoptic GABA receptor occupancy result in disruption of pulsatile LH secretion in the ovariectomised rat. This suggests that GABA neurones provide a tonic input important for the functional integrity of the neural network controlling LH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Electrophysiological evidence for connections between septal neurones and the supraoptic nucleus of the hypothalamus of the rat

In order to see whether septal neurones are connected to the hypothalamic neurones secreting vasopressin or oxytocin, neurones in different regions of the septum were recorded during electrical stimulation of the supraoptic nucleus. The position of the stimulating electrode within the latter was verified using lactating rats in which milk ejections could be induced by a train of electrical pulses applied to the nucleus. The responses of septal neurones to single pulse stimulation were then analysed by post-stimulus time histograms. In the septum ipsilateral to the site of stimulation, 42% of the neurones were antidromically invaded, 20% were orthodromically excited and 21% were inhibited following suraoptic stimulation. In the contralateral septum, 2% of the cells tested were antidromically invaded, 3% were excited and 16% inhibited. In the medial septum, 14% of the neurones were orthodromically excited, and 48% were inhibited. These results provide electrophysiological evidence for direct connections between septal neurones and the ipsilateral supraoptic nucleus of the hypothalamus, and give further support to the hypothesis of a septal influence on the activity of vasopressin- or oxytocin-releasing cells in the magnocellular system.

Electrophysiological analysis of pathways connecting the medial preoptic area with the mesencephalic central grey matter in rats

1. An electrophysiological study of ascending and descending connexions between the dorsal raphe region of the mesencephalic periaqueductal grey matter and the medial preoptic area has been performed in dioestrous female rats anaesthetized with urethane. 2. Extracellular action potentials recorded from 208 neurones in the medial preoptic area were analysed for a change in excitability following stimulation of the periaqueductal grey matter. 174 neurones were also tested for changes in excitability following stimulation of the mediobasal hypothalamus. 3. Stimulation of the periaqueductal grey matter at 1 Hz was rarely effective, but short trains of pulses (three at 100 Hz) usually caused an initial inhibition (62.5% of 208) of both projection identified and adjacent neurones of the medial preoptic area, at latencies of 5--90 msec (mean 34.1 +/- 1.4 msec). Inhibition following stimulation of the mediobasal hypothalamus occurred less frequently (34%) and at shorter latency (mean 12.0 +/- 1.8 msec; n = 48). 4. Less frequently (10.6%) periaqueductal grey matter stimulation caused an initial excitation of preoptic neurones at latencies of 15--180 msec, (mean 35.3 +/- 7.2). Initial excitation following mediobasal hypothalamus stimulation was stronger, occurred more frequently (29%) and at shorter latencies (range 3--60 msec, mean 13.1 +/- 1.5). Following such initial excitation, inhibition of spontaneous or ionophoretically evoked activity occurred more frequently following mediobasal hypothalamic stimulation, than after periaqueductal grey matter stimulation. 5. Twenty-four neurones displayed antidromic invasion following periaqueductal grey matter stimulation. Latencies for invasion ranged from 13 to 50 msec (mean 25.5 +/- 2.0 msec) and are suggestive of an unmyelinated projection. Occasionally an abrupt decrease in latency followed an increase in stimulus intensity. Antidromic invasion from mediobasal hypothalamus was characterized by a shorter latency (mean 12.5 +/- 0.7 msec; n = 43). A period of reduced excitability lasting 40--100 msec followed antidromic invasion from either site. 6. Antidromic responses to paired mediobasal hypothalamic or periaqueductal grey matter stimuli at 5 msec intervals revealed an increased latency of invasion of the second response, due to the partial refractory period of the neurone. Five cells showed a decreased latency of invasion at stimulus separations of 10--150 msec, interpreted as evidence of a supranormal period. Changes in conduction velocity during the supranormal period may give rise to a variable latency of invasion of spontaneously active cells. 7. These results provide evidence for direct, reciprocal connexions between the midbrain central grey and the medial preoptic area. These circuits may play a role in controlling neuroendocrine and behavioural aspects of reproductive functions.