<PAGE>   1
                                                                  EXHIBIT 10.40

                            Corporate Leasing & Syndication Group
                            General Electric Capital Corporation
                            4 North Park Drive, Suite 500, Hunt Valley, MD 21030
                            410-527-9300
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                               February 20, 1997

Mr. James Engelman
Controller
SIBIA Neurosciences, Inc.
505 Coast Boulevard, South
Suite 300
La Jolla, CA  92037-4641

Dear Mr. Engelman:

We are pleased to advise you that our nominees, Mellon US Leasing and
NationsCredit Commercial Corporation, (the "Lessors"), have approved firm
commitments for lease lines to SIBIA Neurosciences, Inc., (the "Lessee"), to be
funded by our nominees, on the following terms and conditions:

1.      TRANSACTION:  The transaction is structured as a true lease in which the
        Lessor will be entitled to claim and retain all of the tax benefits
        associated with ownership of the equipment.  The lease will be a net
        lease in which the Lessee will be responsible for all expenses relating
        to the equipment and the transaction, including equipment maintenance,
        insurance coverage, payment of personal property taxes, recording fees
        and other expenses.

2.      LESSORS:        (A)  Mellon US Leasing
                        (B)  NationsCredit Commercial Corporation

3.      LESSEE:  SIBIA Neurosciences, Inc.

4.      EQUIPMENT:  New Laboratory Equipment, all acceptable to Lessor.

5.      DELIVERY:  All Equipment must be delivered, accepted and scheduled on or
        before March 31, 1998.

6.      ACQUISITION COST:       (A)  $1,000,000.00
                                (B)  $  500,000.00

7.      TERM:  Four (4) years from the Base Lease Commencement Date.

8.      BASE LEASE COMMENCEMENT DATE:  April 1, 1997; July 1, 1997; October 1,
        1997; and December 31, 1997.

9.      BASE LEASE RENTAL PAYMENT:  Lessee will be required to make forty-eight
        (48) monthly rental payments, each payable in advance and equal to the
        following percentages of the Acquisition Cost.

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Mr. James Engelman
Page Two
February 20, 1997


                                               MONTHLY           BASE LEASE
        DELIVERY DATE                     LEASE RATE FACTOR   COMMENCEMENT DATE
        -------------                     -----------------   -----------------
        January 1 to March 31, 1997             2.3305%       April 1, 1997
        April 1 to June 30, 1997                2.3172%       July 1, 1997
        July 1 to September 30, 1997            2.3038%       October 1, 1997
        October 1 to December 31, 1997          2.2941%       December 31, 1997

        These Base Lease Rental Payments were calculated using current money
        market rates; however, money market conditions at the date of funding
        will control the final Base Lease Rental Payment that is fixed for the
        Term.  See "Adjustments to the Base Lease Rental Payment" on the
        following page.

9a.     INTERIM RENTAL PAYMENT:  From the funding date to the Base Lease
        Commencement Date, the Lessee will be required to make Interim Rental
        Payments equal to the daily equivalent of the Base Lease Rental Payment.


10.     TAX BENEFITS:  Depreciation deductions arising out of the ownership of
        the Equipment will be for the account of Lessor and will be recognized
        over a five (5) year period on a 200% declining balance switching to a
        straight-line (DDB/SL) formula using the half-year convention.  A
        Federal corporate tax rate of 35% for 1997 and thereafter was assumed in
        calculating the Base Lease Rental Payment.  In the event of a change of
        tax law or in the interpretation of tax law from the assumptions herein,
        the Base Lease Rental Payment will be adjusted to preserve the Lessor's
        economics.

11.     ADMINISTRATIVE FEE:  GE Capital Corporation acknowledges receipt of an
        Administrative Fee.  This Administrative Fee is considered earned with
        the issuance of this commitment letter and is non-refundable.  Upon
        funding, it shall be applied to the first monthly rental payment.

12.     LESSEE OPTIONS AT LEASE EXPIRATION:  Lessees will have the following
        options:

        LESSOR:  MELLON US LEASING
        At the end of the Initial Term, provided there is no default under the
        lease, the Lessee shall have the option to:

        A.      Renew the lease for an Extension Term of twelve (12) months at a
                monthly rental amount of 1.0% of the Acquisition Cost, in
                advance, after which time the Lessee may:
                1.  Return all the Equipment to the Lessor;
                2.  Renew all the Equipment under the Lease at a term and rate
                    to be negotiated by the parties based upon the then
                    remaining life and the Fair Market Value of the Equipment
                    and the then current money market; or
                3.  Purchase all the Equipment at its then Fair Market Value.

        B.      Purchase all of the Equipment for the greater of ten percent
                (10%) of the Acquisition Cost or its then Fair Market Value.
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Mr. James Engelman
Page Three
February 20, 1997



        LESSOR: NATIONSCREDIT COMMERCIAL CORPORATION

        At the end of the Initial Term, provided there is no default under the
        lease, the Lessee shall have the option to:

        A.  Renew the lease for an Extension Term of nine (9) months at a
            monthly rental amount of 1.2% of the Acquisition Cost, in advance,
            after which time the Lessee may:

            1.  Return all the Equipment to the Lessor;

            2.  Renew all the Equipment under the Lease at a term and rate to be
                negotiated by the parties based upon the then remaining life and
                the Fair Market Value of the Equipment and the then current
                money market; or

            3.  Purchase all the Equipment at its then Fair Market Value.

        B.  Purchase all of the Equipment for the greater of ten percent (10%)
            of the Acquisition Cost or its then Fair Market Value.


13.     ADJUSTMENTS TO THE BASE LEASE RENTAL PAYMENT:  The Base Lease Rental
        Payment stated above reflects current money market rates as indicated by
        the yield to maturity of 5.70% (the Reference Yield) as shown in the
        December 4, 1996, issue of "The Wall Street Journal" for the U.S.
        Treasury Note having an 7.75% coupon and maturing in November of 1999.

        The table below sets forth the U.S. Treasury Notes with similar then
        remaining lives to maturity which will be used to establish the final
        Base Lease Rental Payment, depending upon the date of funding:

<TABLE>
<CAPTION>
        --------------------------------------------------------------
                                            Applicable Treasury Note
                                            ------------------------
            Date of Funding                  Coupon      Maturity
            ---------------                  ------      --------
        <S>                                   <C>      <C>
        January 1 to March 31, 1997........   7.125%   February, 2000
         April  1 to June 30, 1997.........   6.25%    May, 2000
          July  1 to September 30, 1997....   8.75%    August, 2000
        October 1 to December 31, 1997.....   5.625%   November, 2000
        --------------------------------------------------------------
</TABLE>

        The Base Lease Rental Payment actually used will be that stated above,
        increased or decreased basis point for basis point in the implicit rate
        for each basis point change in the yield to maturity of the Applicable
        Treasury Note from the Reference Yield. The yield to maturity of the
        Applicable Treasury Note used to calculate the adjustment to the Base
        Lease Rental Payment will be the yield quoted in the most recently
        published issue of "The Wall Street Journal" on the date of funding.


These firm commitments have been rendered in express reliance on the financial
or other statements respecting the conditions, operation, and affairs of the
Lessee, or respecting the equipment to be leased which Lessee has previously
provided to us, and is based on the understanding that Lessee has committed to
complete the transaction with us and our nominees. The Lessor's commitment is
subject to the condition that there shall be no material adverse change in
either (i) the business or financial condition of the Lessee or (ii) proposed
Federal tax law, prior to any funding under the lease. In addition, fundings
under this lease will be for a minimum of $50,000 each.

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Mr. James Engelman
Page Four
February  20, 1997



An express condition of this commitment is that documentation reasonably
satisfactory to our and our nominee's counsel be executed prior to funding.

This firm commitment will expire on March 4, 1997, unless you acknowledge your
receipt hereof and acceptance by executing the enclosed copy of this letter and
returning it to us by that date.

                                              GECC Capital Markets Group, Inc.



                                              By: /s/ LOUIS J. VIGLIOTTI
                                                 -----------------------------
                                                      Louis J. Vigliotti
                                                        Vice President



ACCEPTED BY:
SIBIA Neurosciences, Inc.

By:  /s/ WILLIAM T. COMES
   ------------------------------

Title: President and CEO
      ---------------------------

Date:  March 4, 1997
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